Brakes

Got a strange one here!

I've done alot of brake work but this has me stumped.

If I apply the brake pedal very slowly the pedal goes to the floor. This happens so intermittantly (about 1 out of 100 slow stops) that it has been hard to diagnose. There is no leakage of fluid at all. I have bleed them at the calipers and found no air (the car has 60K miles). It does it hot or cold. If I apply the brakes firmly they stop fine. If I apply the brakes hard, the ABS works perfectly. I replaced the pads about 6 months ago with no problems until now. All calipers are in good working order. I don't have alot of experience with ABS so that's my weak spot. I have asked a few mechanics only to find them perplexed as well.

Got any ideas??

if you are sitting in traffic or at a light does the pedal slowly go to the floor ?

if you are sitting in traffic or at a light does the pedal slowly go to the floor ?

I e-mailed you with a response. I'm new and not sure how to use this forum yet. To answer your question again, the answer is no. After a slow stop and this occurs you can immediately apply the brakes and they're fine with no fade, and again all fast or hard braking is fine (I know what you're thinking -- Then only do fast - hard braking -LOL).

Here are a couple checks you can do:
With the engine off pump the brakes with normal pressure a few times
Press pedal hard & hold - If it sinks then a brake line, master cylinder, or wheel cyclinder is bad.

Start engine with brake pedal pressed with normal pressure
If the pedal height does not change then brake booster or check valve is bad.

To test the check valve disconnect booster vacuum hose
Start engine & check for vacuum
If no vacuum then the check valve is bad

Never pump the brakes up too much - you can blow out seals

What color is your brake fluid & have you added any because it was low?

same deal happened with our old bimmer. it was the master cylinder. check that out.

Here are a couple checks you can do:
With the engine off pump the brakes with normal pressure a few times
Press pedal hard & hold - If it sinks then a brake line, master cylinder, or wheel cyclinder is bad.

Start engine with brake pedal pressed with normal pressure
If the pedal height does not change then brake booster or check valve is bad.

To test the check valve disconnect booster vacuum hose
Start engine & check for vacuum
If no vacuum then the check valve is bad

Never pump the brakes up too much - you can blow out seals

What color is your brake fluid & have you added any because it was low?


I really appreciate your input! I wiil do what you suggest. I had bled the system several months ago when I changed pads. Since the problem started I bled all wheels again and didn't find any air. The fluid is new so the color is clear (I only use Valvoline Syntech). If a line or wheel cylinder was bad wouldn't it leak??

THANKS -- Pat
THANKS AGAIN --- Pat

I really appreciate your input! I wiil do what you suggest. I had bled the system several months ago when I changed pads. Since the problem started I bled all wheels again and didn't find any air. The fluid is new so the color is clear (I only use Valvoline Syntech). If a line or wheel cylinder was bad wouldn't it leak??

THANKS -- Pat
THANKS AGAIN --- Pat


It sounds like a master cylinder problem. The piston seal may be weak or damaged, letting fluid squeeze through the release hole in the piston under soft braking. I had the same problem on one of my motorcycles (talk about dangerous?!) and replacing the master cylinder fixed the problem.