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1991 GMC k1500 4X4 350cid 157000miles

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New User

Nov 28, 2012, 11:04 AM

Post #1 of 6 (1636 views)
1991 GMC k1500 4X4 350cid 157000miles Sign In

I have a GMC k1500 1991. my brake fluid turned black and my drivers rear brake was leaking. I replaced the cylinder on the rear brake and sucked out all the fluid from the master cylinder. I bled the brake lines to get clean fluid in them. everything seems to be working except when I step on the brakes it goes all the way to the floor and my brake warning light comes on. I can pump them twice and the light goes out and the pressure is back and will stop just fine. but take off from the stop and within 30 seconds if you step on brakes they go all the way to the floor again. again they pump up fine. but it is like my master cylinder is not holding the pressure?

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Nov 28, 2012, 11:26 AM

Post #2 of 6 (1626 views)
Re: 1991 GMC k1500 4X4 350cid 157000miles Sign In

Did it get contaminated with something? Is any of the rubber swelled like in the cap?


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.

New User

Nov 28, 2012, 11:30 AM

Post #3 of 6 (1621 views)
Re: 1991 GMC k1500 4X4 350cid 157000miles Sign In

I am not sure of contamination. I purchased the truck back in 1997 and the fluid was black at that time. I decided this last year since I was replacing the rear cylinder I might as well clean it out. so I used a turkey baster to suck out the old fluid and replaced it and bled all the lines to get clean fluid out. there is no swelling of the rubber parts ie cap or seals. it does not lose any fluid.

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Nov 28, 2012, 12:13 PM

Post #4 of 6 (1603 views)
Re: 1991 GMC k1500 4X4 350cid 157000miles Sign In

Hmmm - just the one leaking wheel cylinder. Not the exact case but do things in pairs.

Black fluid for a long time. Who could know now if you didn't save any. Pure even dirty brake fluid would wash off with plain water, if an oil in it, it wouldn't and be a nightmare that should have happened long ago.

With the pedal to the floor my guess is master cylinder tore up seals inside just bleeding for that fix and that it has traveled to bottom where it hadn't been.

Fluid does get dark over time but I wouldn't call it black?


Veteran / Moderator
DanD profile image

Nov 28, 2012, 12:40 PM

Post #5 of 6 (1593 views)
Re: 1991 GMC k1500 4X4 350cid 157000miles Sign In

If you’re 100% sure that all the air has been bled out and there are no leaks; you’ll want to confirm whether the problem is at the wheel(s) or the master.
You can do this by clamping off all the flex lines with vice grips; being careful not to damage the flex lines; all you need to do is pinch them closed. Now try the pedal; if it is still soft with excessive travel, the issue is likely a bad master cylinder. If the pedal is hard and to the top; have a helper hold pressure on the pedal as you release the grips one at a time. When the pedal drops you’ve at least found the wheel or wheels that are at fault. From there you’ll have to determine why; out of adjustment, seized caliper guide pins or pads seized in their sliders.
Last thought and I haven’t confirmed whether this vehicle has only RWAL but if it does, there may be an issue with the ABS unit on the frame rail. Where some crap from you flushing the system has got into the units dump valve and every time you pump the pedal you’re actually filling the units accumulator before you get a solid pedal. Those ABS units were not serviceable; so it would mean replacing it or by-passing it. Most RWAL systems of that era have long crapped out so by-passing it would be a valid solution if it is the issue.


Canadian "EH"

New User

Nov 28, 2012, 1:30 PM

Post #6 of 6 (1579 views)
Re: 1991 GMC k1500 4X4 350cid 157000miles Sign In

I have one question.. is there suppose to be little steel bb's in the bottom of the master cylinder? I have 2 k1500's and the one I did not change the fluid on I noticed there was a little bb in teh bottom of the master cylinder. I still have the old fluid from the change so I can search though it with a magnet if I need to. but I believe I still had the bb in the one with a problem. but do I have it in the right place? would be my next thought.

thanks guys

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