[Lawrence]

Recently my brake pedal gets soft while driving. This only happens occassionally and once I pump it a few times it's fine. Sometimes this will happen 2 or 3 times while driving. I've bled the brakes at the wheel cylinder and don't have any fluid leak as my resevoir is always full. Any ideas ? Master cylinder ?

[GT182]

Check the rubber seal under the MC cap. Might not be sealing right and is letting air in.

[guccieng]

replace the master cylinder NOW (don't drive it). i've had one go out on the highway. not fun!

[formula400]

agree. if there is no leaks, replace that master cylinder

[chrisp]

Pull off all 4 wheels , peel back rubber dust seals from each wheel cylinder . If you see fluid , bad wheel cylinder , if no signs of wetness , inspect all lines & fittings , if all is good replace master cylinder . I had done a full system replacement and had a similar problem , no visible leaks no fluid loss , but 1 end of 1 wheel cylinder under the dust seal was wet . Seems it was pushing a bit out and drawing it back in upon pedal release . Yet no external signs of leak .

[geeteeohguy]

It sounds like your master cylinder is internally bypassing. You will lose pedal pressure but not fluid. I agree with the others: if you have no leaks at the lines or wheels, replace that master cylinder.

[Greg Reid]

Originally Posted by chrisp

Pull off all 4 wheels , peel back rubber dust seals from each wheel cylinder . If you see fluid , bad wheel cylinder , if no signs of wetness , inspect all lines & fittings , if all is good replace master cylinder . I had done a full system replacement and had a similar problem , no visible leaks no fluid loss , but 1 end of 1 wheel cylinder under the dust seal was wet . Seems it was pushing a bit out and drawing it back in upon pedal release . Yet no external signs of leak .

I had the same thing on my VW. Couldn't figure out where the fluid was going. Never hit the ground and all wheels and tires were dry on the inside...but I was losing it. It was just wet under the boot.

[Lawrence]

Thanks everyone... What a wealth of knowledge.

[Tim john]

One additional thing to consider is your fluid. If it is old it has most likely acquired moisture from the heat cycles of braking (internal friction within the lines) so at the very lest replace all the fluid in your system too. Flush and re fill with quality fluid.

Tim john---

[Old Blue 66]

Originally Posted by guccieng

replace the master cylinder NOW (don't drive it). i've had one go out on the highway. not fun!

Same here on a 64 Bonne. And lost it on my current car while driving Woordward in Detroit behind a VERY expensive modified GTO. I got lucky twice.

Replace the master. You can never be too safe when it comes to brakes.