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Question about Power Brake Booster

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Mazda Carnage

Jan 6, 2017, 2:13 PM

Post #1 of 13 (1346 views)
Question about Power Brake Booster Sign In

I have a question I can't find a proper answer to.
I understand how a brake booster works and know everything mechanical like the back of my hand, for the most part.
Tried calling some aftermarket brake booster sales company techs but didn't really get an answer to my question.

I have worked on all things car related for many years but never have I experienced this problem, seen it happen or found anybody with info about it.

This would be the workings of about every single diaphragm automobile brake booster:

My question is (how best to phrase it).
To simplify:
(Can a proper functioning brake master act like a foot pump filling an air mattress).
- Say you undo the brake master cylinder from the brake booster on a car (so there is full pedal travel).
- You started the engine and pump the pedal to the floor repeatedly while the engine idles.
- Would the Brake Booster be pumping/adding un-metered air into to the idling engine, (after the throttle body) to the point of leaning out and almost stalling the engine .
- I realize engines idle on very little air (idle air holes are very small), so if the amount of air the engine is consuming from the idle bypass in the throttle body is smaller than the amount of air getting passed the diaphragm in the booster when the pedal is released, then pressing the pedal with the diaphragm sealed would result in pumping air into the booster vacuum hose and check valve to the engine (If the engine vacuum is say 19-20in hg and the pressure in the booster diaphragm is any closer to atmosphere). And otherwise at the very least the booster would be giving the engine a steady supply of un-metered air to suck out of it.

Would a properly functioning Vacuum Power Brake Booster do this (with full pedal travel)?

The above symptoms where noticed when the brake system on my car failed. The symptoms, no fluid loss, no leaks, good pads, disks and calipers all around. If you pumped the pedal with the engine of it stayed stiff, If you start the car and pump the pedal it stayed stiff, if you used the brakes when driving the pedal would go to the floor. This made it impossible to repeat the problem during trouble shooting until suspecting one of the calipers for leaks by cleaning it, raising it of the pads and putting a block of wood in it, the extra travel in the piston initiated the pedal falling to the floor, as I pumped the pedal over and over I noticed the air fuel ratio's getting leaner and leaner right to 20.9 a/f if I pump fast enough.
This lead me to suspect it was the brake booster, so I swapped it, which only made the problem worse (stronger booster) and I realized that it must be the seals in the brake master failing to hold fluid pressure after a certain amount of travel with the pressure added by the booster ( +- 10X pedal force).

The rear bleeders had both rusted off the car and I didn't want to have to swap the master or bleed the system, after the booster swap I had no choice, I installed a used master that was working perfectly when I removed it, I tried bleeding at the rear lines... During the bleeding process I started the car to check the pedal it was soft but did stop before the floor and I noticed the same lean condition with lots of pedal travel. Last night (in the freezing cold) I extracted the broken bleeders and installed others, so I can now properly bleed the system I also remembered I have a new brake master cylinder at my shop and it's not good for the seals to make the pedal travel to far on an old master cylinder even if it's working well, so I am going to get the new master and bleed the whole system properly.

My concern is the Brake booster should it be putting that much air in the engine under these circumstances.

And is it possible that the master failed real slow (for over a year) without any real symptoms aside from a tiny bit more travel, resulting in a tiny bit more air getting into the engine every time the brakes where pumped coming to a light, causing a moment of rough idle?
Because since all these brake problems started I used the handbrake and down shifted to stop a few times and used the really low pedal brakes to stop the car and the rough idle is caused by the brake pedal (not if you push it just enough for the lights to come on but if you try braking with it).

I have felt like there is and been looking for a vacuum leak on this car for over a year now, I am positive that once I install the new brake master and bleed the system properly the brakes will be restored but I don't know if the Vacuum/idle issue will be gone, I brought home 2 brake boosters from cars that had good brakes, I tested the one I thought was failing and compared the two I brought home, they all seemed pretty much the same, I used the one that seemed the strongest. (They all hold vacuum).

Wanted to add this is a no ABS brake system, just: Booster -master -2 lines -divider -4lines -4calipers.

(This post was edited by Mazda Carnage on Jan 6, 2017, 2:20 PM)

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Jan 6, 2017, 3:47 PM

Post #2 of 13 (1336 views)
Re: Question about Power Brake Booster Sign In

I couldn't even read all that gibberish.
You have way too much time on your hands.
The booster doesn't push air. It is pulled by the suction of vacuum when triggered by the pedal.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

Ultimate Carjunky / Moderator
Discretesignals profile image

Jan 6, 2017, 4:51 PM

Post #3 of 13 (1332 views)
Re: Question about Power Brake Booster Sign In

Yes, if you keep pumping the brake pedal rapidly while the engine is idling, you will create a lean condition. I've never seen someone pump the brake pedal with the master removed though.

Brake booster malfunction won't cause the brake pedal to go to the floor. Pedal going to the floor is usually from a hydraulic issue such as bad master, leaks, trapped air, faulty ABS hydraulic modulators, etc. It is possible for a brake booster to malfunction in a way that causes it to apply the brakes, but that is really rare. Most brake boosters fail when the valve inside doesn't seal allowing a large vacuum leak when you push on the brake pedal. Some will fail if they get contaminated with brake fluid that leaks from the master cylinder seal.

Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.

(This post was edited by Discretesignals on Jan 6, 2017, 5:00 PM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jan 7, 2017, 12:39 AM

Post #4 of 13 (1308 views)
Re: Question about Power Brake Booster Sign In

OP - I thought I was the "verbose" one of this site! You win.

Vacuum boosters are just that. Boost foot pressure with a vacuum diaphragm which by design should hold vacuum even for one or more assisted braking AFTER more vacuum can be added.

They are NOT generally fixable except a check valve and grommet otherwise if bad you get a new or rebuilt one.

That just on the booster not the rest of your braking problems,


Mazda Carnage

Jan 30, 2017, 9:10 PM

Post #5 of 13 (1261 views)
Re: Question about Power Brake Booster Sign In

The new master cylinder I had at my garage was gone, just an empty box remained.
I tried 2 used master cylinders then ordered a new one rush delivery from Rockauto.
Still the pedal drops to the floor.
I welded a brake line and fitting shut to test the back lines, the plan was to block the rear passenger line at the proportioning valve, if I got a pedal it was that line if not I would block the main master line for the rears and if I got a pedal it was the drivers rear line. The factory workshop manual doesn't specify the proportioning valve layout, but a dealership site listed the passenger side master cylinder hard pipe as rear main pipe, Unfortunately the master lines/proportioning valve divides FL/RR FR/RL. I made a venturi brake bleeder (uses compressed air to draw fluid from the lines) I works really well but the small pancake compressor I am using outside builds up ice in the cold weather and blocks the air gun I am using to draw fluid.
instead of bleeding over and over I just installed 2 new rear brake lines, when connecting the rear drivers line to the rubber flex hose some rust must have dislodged at the end of the flex hose and it blocked the small hole where the flex line mounts to the strut, couldn't blast it out with air or poke through it with a thin metal rod so I swapped all four rubber flex lines for some new braided stainless lines I had in the basement. While doing the rear lines I rebuilt all four working/not leaking calipers, they where due for a rebuild, especially the rears with the handbrake mechanism.
Still the pedal drops.

I welded a second fitting plug and blocked both fittings at the master cylinder and get a hard pedal, I bought and bent a 30" and 40" brake line to replace the front and as I was disconnecting the 6 lines at the proportioning valve I decided I am going to change that also. Last night I grabbed to used proportioning valves at my garage with main master lines, I bought a 20" line and bent it to match one of the main lines and chose the cleanest one to replace the other.
I have not had time to install the front lines and proportioning valve yet and I know none of the lines I want to replace are leaking.

I am now certain its the proportioning valve that has failed, similar to this:
(link to another site deleted - said - "brake-problems-don-t-overlook-the-proportioning-valve"

(This post was edited by Hammer Time on Jan 31, 2017, 6:09 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jan 31, 2017, 3:46 AM

Post #6 of 13 (1251 views)
Re: Question about Power Brake Booster Sign In

No links please - left the suggestion is enough.
Soft brake pedal: Master cyl not functioning and or rod from/thru booster isn't right distance. How - wrong rod or bent/broken something where pedal pivots.
Back a few you wondered or wanted to know if a master cylinder could pump up an air mattress!!!! Yes but would ruin seals in no time and end that very slow use of one. Not much volume per push.

If you don't bench bleed a master cylinder FIRST it will trap air. Some applications it will get over that some/most won't.
Hey - master must have some pre-lube of brake fluid new or reman if legal (not here) or it would tear itself the shreds inside right away.

What is this thing? Many vehicles DO NO BLEED in the order you listed meaning the longest line first you think but may not be or be crossed as in FR and LR are one isolated hydraulic system from the other two.

Vacuum bleeding? IDK - pressure bleeding is the way with problem systems to bleed all air out - that's an opinion. I've only used vacuum on fussy systems to get lines started once something failed must have had a blockage then behaved.

Google/search out how a proportioning valve works and self centers itself. If it can't you bleed just the side blocked off in hopes it goes back to center. If not only one system will work and would make a soft pedal likely just partly working. Most throw a brake warning light if and you are done as a good clue and it's VERY rare to almost never,


(This post was edited by Hammer Time on Jan 31, 2017, 6:09 AM)

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Jan 31, 2017, 6:08 AM

Post #7 of 13 (1245 views)
Re: Question about Power Brake Booster Sign In

I don't believe a bad proportioning valve can cause no brake pedal.

PS, please don't change the title of a thread. I changed it back in both responses.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

(This post was edited by Hammer Time on Jan 31, 2017, 7:38 AM)

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jan 31, 2017, 6:37 AM

Post #8 of 13 (1231 views)
Re: Question about Power Brake Booster Sign In

Totally agree wouldn't cause "no" brake pedal rather make it hard all the time if totally jammed up. Said NEVER needed to replace in now decades on end. Rusted lines - hunting down right flare nuts if trashed - but they worked or all unstuck.
Could be just chance as they do list them for sale and think many are for custom work?


Mazda Carnage

Jan 31, 2017, 6:41 PM

Post #9 of 13 (1209 views)
Re: Question about Power Brake Booster Sign In


Used the attach link feature, couldn't see in in the text preview so I added it.

I bench bled each master cylinder I installed on the car. The booster rod is adjusted to factory spec.
With two plugs in the master (no lines) the pedal is rock hard and stays hard with the engine running. So the booster and master are good.
I googled 1988 mazda 626 dual proportioning valve and found nothing.
The only sensor on this brake system is for the reservoir level.

The venturi bleeder works very well I can pull 1 liter of fluid through a line in under 2minutes, after using it on all four brakes I bleed manually (using a pipe and the drivers seat to hold the pedal) to check for air and there is none.

I got the proportioning valve off last night, it has a lot of corrosion on the back side, it's -18C out which slows things down, I can only hold the tools for 5minutes before my hand goes numb and I drop them + having my arm stuffed between the intake manifold and firewall makes it hard to work.

After work tonight I will install the replacement valve, bleed the system and see if the brakes are back.

I only need another month or two from this car, I was about to start the body work on the car when I found a 1988 626DX for sale with no rust, the car was from out west, I bought it for 800$ and will transfer everything over to it from the car I am driving now (engine, exhaust, interior, wiring..), but first I will remove all the tar from the floors and wheel wells and coat the whole bottom side and rockers in POR15 or rhino lining, they use lots off salt here in Quebec and running a high boost set up seems to salt blast the car away no matter how good the paint is and clean the metal is it only takes 2winters and the rockers are gone, larger polyurethane mud flaps should also help.


Hammer Time sorry about changing the tittle, I didn't know I shouldn't, how do I make a post in this thread without (in this case) replying to myself or anyone else, I can't find new post, only option seems to be to reply, starting every post with Re: and if I post anything after my original post it makes no sense for me to reply to myself and looks wrong. (I know you guys didn't design the forum and got used to it, but when I search the internet looking for info, whenever I would come across a forum where every post is a reply including updates by the thread starter I personally think it's bad forum format and shortsightedness on the part of the developers by assuming the OP will only post once in the whole thread).


Hammer Time, Tom Greenleaf, I think just like you both when it comes to the proportioning valve, I have been working on cars for 18years and I have never seen a proportioning valve fail or through in any way shape or form it could cause a soft pedal, but it's the only part of the brake system left to change.
Just like in the now removed link, the proportioning valve was the cause of a soft pedal and the last thing they suspected so the last thing they changed.

And because I also don't think a proportioning valve would cause a soft pedal it's the last thing I looked at.


Tom Greenleaf, the bleeding order for this car is RR, RL, FR, FL that changes if the car has ABS, but I go around and double bleed all the lines because I have been working at the master cylinder before the proportioning valve. I am self tough but have rented a garage since 1999, I have been surrounded by veteran mechanics in neighboring shops form many years and surpassed most of them in two years (not in diagnosis experience but in skill sets), learning to port and polish, rebuild engines transmissions, alternators, powder coat, fabricate brackets and mounts, repair stripped key ways, repair and rebuild almost anything, great with wiring diagrams and pinouts and build my own harnesses, tuning engines, I understand sensors their signals (volt or Ohm, low to high or high to low and in what range 0-1v 1-5v...) I am also very good at diagnosing almost any problem, the only things I cant do with a car is align it perfectly by eye and feel (but I get it pretty darn close) and figure out the problem with the brakes on this car.
My shop has two 1989MX6 shells in it so I can't do work in it now, the guy with the shop next to me only does brakes and sometimes gets me to help him bleed them properly (I am really good at bleeding brake and clutch systems).
All this to say if I give up working on my car in -20C weather and send the car to the neighboring shop there is no way I would send it with a pin sized air bubble in the system because I would never hear the end of it. I am 1000% sure there is no air in the system and it has not been the cause of this problem.

My 1989 Mx6 GT engine:

The fact that all four calipers worked and not a drop of fluid was leaking is what lead me to suspect the booster/master end of the brake system and made this problem very hard to diagnose. None of the flex hoses where stretching and without the booster assist (engine running) all four brakes locked the disks hard.

(This post was edited by Hammer Time on Feb 1, 2017, 6:32 AM)

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Feb 1, 2017, 6:50 AM

Post #10 of 13 (1195 views)
Re: Question about Power Brake Booster Sign In

Look, just simply use the reply button.

This format may not be perfect but it works just fine. We don't need you to use the quote to know what you are responding to. All it does is confuse your response so we have to figure what is just repeated. If you are concerned about forum etiquette , make sure you post the year, make, model and engine size in your original question.

A proportioning valve can fail by directing pressure to the wrong wheels but unless it is leaking, it's not going to cause no pedal.

How about you go back and confirm the calipers are on the correct side by making sure the bleeder is higher than the feed line on each caliper.


We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

Mazda Carnage

Feb 10, 2017, 2:20 PM

Post #11 of 13 (1148 views)
Re: Question about Power Brake Booster Sign In

Hammer Time I didn't post the year, make, model or engine size in the original question because it was a general automotive question about power brake booster's on gasoline engines.

I have heard of people reversing calipers and never getting them bled properly.

These calipers have one slider and one slider sleeve, unless someone mixes up the carriers then the calipers cant go on backward.
They are also labelled R L, but the position on the bleeder hole is in the top center of the caliper and angled, If the calipers got reversed the bleeders would all still be at the top of the caliper but they would point at the struts.

After trouble shooting forever and slowly working my way through the system, I just swapped everything that was left.
I replaced (had most, purchased brake master and 3 lines):
-All 6 brake lines.
-New master.
-The proportioning valve.
-Completely rebuilt the rear calipers.
-New loaded front calipers (with pads and brackets).
-Swapped brake booster.
-New fluid.
-System bled to perfection.

Still the pedal was soft and low. It goes almost to the floor and locks the brakes, you have to be delicate with the pedal to slow the car or it just locks the wheels.

I finally called the mechanic with the shop next to mine, he has been 40+ years in the trade, for the last year he decided he only wants do do brakes.
He looked the brakes over and test drove it, checked the lines, fittings, caliper movement, tried bleeding. He felt there was too much braking all at once. He didn't charge me anything and went home to think about it.

My to try list was:
-Bleeding the Rear right and front left (same master line) together while tapping the proportioning valve (in case any air was stuck).
-Same for the RL / FR .
-Flush the system with brand new fluid, all the fluid was new but had been run through the system many times, with the reservoir opened and it snowing a lot of the time, the freezing weather and the possibility of condensation build up in the glass jar I drained the fluid into there was a tiny possibility of water getting in.
-Adjust the hand brake cable, (had manually adjusted the pistons after the rebuild, screw type, 180degrees notched flat face pistons. -> get them as close as possible, hold the pedal 20 seconds with engine running, adjust had brake cable).
-Quadruple check the pedal to brake booster adjustment.

Through my many internet searches of the symptoms I was experiencing I found thousands of suggestions and instructions in threads, some solutions: rear drum adjustment, bent brake pads, bad bleeding... for the most part no solutions or resolutions where posted.
In one thread there was a discussion about adjusting a brake booster, one mentioned his adjusts behind the master the other guy's adjusted at the pedal.
I have the 1988 mazda mx6/626 factory workshop manual, also the 1989 mx6/626WSM and the 1992 WSM, and the 1988 mx6/626 factory training manual.
The only instructions in any of the WSM about adjusting the booster is at the brake pedal.
While I was adjusting the rear handbrakes and planing the last futile thing on my list I though about a few of the garage customers I had years ago that had hydraulic clutch problems after they performed an auto to 5speed swap, every time it was the master cylinder push rods length or adjustment. In one case it's shortest adjustment still engaged the masters piston making it impossible to get the pedal to pump up.

I pulled the brake master cylinder out of the way and there is/was an adjustment at the tip of the front brake booster rod, I tightened it as far in as it would go (made it shorter, stick out less from the booster into the master), then I adjusted the rod at the pedal to be longer, and finally got the brake pedal back.

Short version:
Resolution to my problem of a constantly soft brake pedal. Make sure the front power brake booster push rod isn't engaging brake master piston when they are bolted together.

In the case of a 1988-1992 Mazda 626/Mx6 and Ford Probe the brake booster has two rods in it, the front rod sits (floats) in a cup in the center of the diaphragm and controls the distance between the master piston and the diaphragm.
The rear booster push rod that sits between the pedal and booster controls the distance between the pedal and diaphragm.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Feb 10, 2017, 3:33 PM

Post #12 of 13 (1144 views)
Re: Question about Power Brake Booster Sign In

Tough read: Check just this > Push the brake pedal after you've spent the vacuum of booster. Don't run engine and push with a finger on the pedal and see how far it travels. That much is called "lash" or the required free area so fluid will return. It too tight you burn up brakes. I too much you have a permanent low pedal and doubt it adjusts how far pushrod pushes before making contact with master cylinder.

That's where the problem is. Now find out where and why that isn't right - worn - wrong parts or just once found a new master improperly made such that centering hole wasn't deep enough so that one was too tight but could have been drilled new too deep as well and have your problem or any excessive "lash!"



Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Feb 10, 2017, 10:07 PM

Post #13 of 13 (1129 views)
Re: Question about Power Brake Booster Sign In

Sorry for a second post but should show if you already read last one and new.

One more whacked thing can happen that causes low pedal not on the list is so rare only heard of it not seen.

In some situations a car could have had brake pedal pushed so hard it bent the firewall! Usually I would say from an accident. 1/4 inch would make a total change but hard to see.

I have witnessed watching boosters move (most will some) a lot too much when brakes are applied hard whole assembly moves and shouldn't. That was such a wreck of a vehicle worn and abused plus a total rust bucket I'll write that off as a junk vehicle not fixable for road use anymore.

Other than that it's as said already. I suggested using your finger because I doubt with force of your finger you would even compress an air bubble still not bled out still possible IMO with this otherwise it has to be wrong amount of "lash" as I put it,


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