I have a 97 Ford F250 7.3L Diesel 4X4 truck with brake troubles When I first apply pressure to the pedal I get some braking but eventually it fades to the point that the brake light comes on. If I try to pump the pedal then it's extremely hard to push down and there's very little braking. I have to push really hard to get any braking. Is this a sign of the diesel brake booster vacuuum pump going bad or is it the master cylinder.Help please

Is the fluid level dropping at all?




78 Corvette Pace Car For Sale-18k mi orig-Florida

Yes the brake fluid is at the same level I don't see any signs of brake fluid leaking.

If your pedal is slowly dropping to the point that it puts the brake light on and it's not losing any fluid, then that indicates a bad master cylinder.




78 Corvette Pace Car For Sale-18k mi orig-Florida

Arggh! Just went thru this with the similar Chev of my own. Raining to beat the band so you can't really tell but in that case fluid was lost! As Hammer said if not loss of fluid it can be the master cylinder alone doing that and hope your 1997 will bleed out without additional headaches. Mine was short areas of brake line rusted out and master survived bleeding process - some won't at the age and add to the costs of course.

IMO - Fords are more brake and fuel line rust resistant than GMs. If rust isn't an issue where you are then as Hammer said this would almost always be a master cylinder problem. If you find that to be the issue - I strongly suggest paying the extra for new over reman ones,

T
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Tom Greenleaf - MetroWest, Boston

Let me add that in some cases it can also be the ABS unit bypassing but that's pretty rare. You always look at the master first.




78 Corvette Pace Car For Sale-18k mi orig-Florida

ford trucks have had a problem for years with the firewall flexing during braking ,causeing symtoms similar to yours. might want to check the area around the brake mounting while someone presses the brake.
LIFE IS SHORT GO FISH

Quote

That isn't going to put the brake light on.




78 Corvette Pace Car For Sale-18k mi orig-Florida

I didn't say it did. i was just stating a fact that could be PART of the problem..Here is some items that can trip a lite.
A red brake warning light that stays on could result from a defective parking brake switch, low brake fluid level, hydraulic leak or an electrical fault. If rear wheels lock during braking you could have grease on the shoes, out-of-round or oversized brake drums, loose wheel cylinders, or your hydraulic control valves could need service.
Finally, for the car that pulls to one side during braking, causes include: a defective suspension, leaking wheel cylinder, plugged brake hose, improper tire pressure, or a stuck wheel cylinder or caliper piston.
LIFE IS SHORT GO FISH



(This post was edited by re-tired on Oct 11, 2009, 3:09 PM)

BTT - the pedal can fade without loss of fluid and would trigger an imbalance and most vehicles would light a RED brake warning light if so. This smacks of a bad master cylinder as Hammer said.

Grease on brakes and many other faults wouldn't cause the pedal to fade! Same light will light if parking brake doesn't fully retract to off in many vehicles. If this was an ABS issue that light should be amber and on!

Frozen caliper or wheel cylinder could after driving heat up brake fluid to boiling and a soft pedal would result. That about always demonstrates itself as a hard pull if a front issue or a hot wheel even smoking hot which was not mentioned so far so I think this is a master cylinder. They can do that without warning. Best we can do is educated guessing unless we have the vehicles in front of us and we do a lot of it here,

T
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Tom Greenleaf - MetroWest, Boston