What is wrong with my brakes?

[techs]

99.999 percent of the time the brakes on my 1999 Ford Explorer work fine. Once in a while, while traveling at a very slow speed and I apply the brakes the pedal sinks and I think I get a feeling like the anti-lock brake system is coming on, but I could be mistake about that part. The care does stop, though I do put extra pressure immediately to make it stop in the distance I wanted to.
I took it to my mechanic and he was unable to re-create the problem, since it happens so infrequently. He checked the anti-lock brake sensor and it is fine.
Could I have a bad piston in my master cylinder? I was thinking if the master cylinder rotates it may have some wear that lines up with some wear on the sleeve???
Any ideas?


Moved to The Garage.

minendo
Forum Administrator

 

[bobsmith1492]

My '89 Bronco II has done the same thing a few times. Usually the engine has been running really slow; I would think the brake pressure is just a bit lower than normal.

 

[techs]

Originally posted by: bobsmith1492
My '89 Bronco II has done the same thing a few times. Usually the engine has been running really slow; I would think the brake pressure is just a bit lower than normal.

Hmm. I thought when the engine runs slower there wouldn't be enough vacuum for the power assist, making it harder to push on the pedal, not the pedal going towards the floor?

Also, noticed the mods moved this to the Garage.
Sorry for the wrong forum.

 

[Zenmervolt]

Depending on how the vacuum system is set up, there may be other vacuum operated accessories (specifically the evaporative emissions purge system) that would activate sporadically (by design) and could theoretically affect the amount of vacuum supplied to the brake booster.

Master cylinder doesn't rotate, so it wouldn't have wear that only sometimes lines up. If the ABS was actually coming on, you would feel a noticeable pulsing/vibrating feedback through the pedal.

My guess would be that this is a quirk of the vehicle and not a failure point, though since brakes are a safety item I would have you master cylinder checked just in case.

ZV

 

[techs]

Originally posted by: Zenmervolt
Depending on how the vacuum system is set up, there may be other vacuum operated accessories (specifically the evaporative emissions purge system) that would activate sporadically (by design) and could theoretically affect the amount of vacuum supplied to the brake booster.

Master cylinder doesn't rotate, so it wouldn't have wear that only sometimes lines up. If the ABS was actually coming on, you would feel a noticeable pulsing/vibrating feedback through the pedal.

My guess would be that this is a quirk of the vehicle and not a failure point, though since brakes are a safety item I would have you master cylinder checked just in case.

ZV

Thanks!

 

[shekondar]

My 2000 Ranger does the same thing, especially in hot weather & heavy traffic. My understanding is that this is what's known as "brake fade", and there isn't really a good way to fix it, other than trying to avoid it by not letting the brakes overheat.

In other words, Ford brakes just suck.