Soft brake pedal

[365gtc/4]

I have a C4 and posted this on the C4 webiste but thought that I would post it here as well. Maybe a Queen's brake system is similar. Anyway what I don' understand is why I had a firm brake pedal, then a spongy brake pedal, then a firm brake pedal and then a spongy brake pedal again all in one days driving.

My brakes are usually pretty good with only 1 inch of brake pedal travel with absolute maximum brake pedal pressure. Today I went for a drive. First hour the brakes were normal and I was NOT using the brakes hard at all. Maybe 1 or 2 reasonably hard stops 10 minutes apart. I used the brakes very gently a few times and all OK, then I got a very soft pedal which went 1/2 way to the floor. On the first pump the pedal returned to normal. This would last for about 1 minute maximum, then I would have a 1/2 way to floor pedal again. If I did 1 pump of the brake pedal it returned to normal again and I would have a firm pedal again for up to a minute, then soft pedal again. This occured about 10 or so times, then DISAPPPEARED. Pedal NORMAL again. No problems for 50 brake applications, then the 1/2 way to floor again. Each time 1 pump of the pedal returned it to normal which lasted up to a minute, then soft pedal again. I checked the brake fluid level and it was normal. I checked all 4 wheels and no fluid leaking. I was NOT using the brakes anywhere near hard enough to cause the fluid to boil. It was also a cool day, maybe 19 degrees C. Funny thing was that half way through the drive the probelm disappeared completey for at least 50 brake applications with a very good firm barke pedal. Any thoughts?? Do I just bleed the brakes as a precaution? But I would like to know the reason for the problem. The brake system was bled, flushed and refilled with Castrol Dot 4 two years and 5000kms ago. I do not use the brakes much at all, but I do use the accelerator a bit !!!

John
.

[330GT]

Sounds similar to a problem I had on my 330.

Soft braking would just have the pedal go down. Hard braking would feel normal. A hard push and then slackening off left a normal feel for soft braking.

I had the master cylinder rebuilt and that fixed the problem. I think that the cup seal was worn. A hard push would flare it out and produce a good seal for that stop, while a soft push would have the fluid bypassing the worn seal. Note, this started happening on a single drive, no previous indications of a problem.

[365gtc/4]

Thanks Kerry. That might be the answer. I can't recall if I had a spongy pedal if I braked hard. I certainly did when I braked slowly. I will take the car out and brake hard to see if I get a spongy brake pedal. Problem is of course it is intermittent. I don't really want to just bleed the brakes to see if that fixes the problem. If the fluid boils the gas bubbles stay there for a long time before they get re-adsorbed. And my issue was intermittent over a 1 hour period. Not enough time for the gas to get re-adsorbed. Then come back again with me NOT doing any hard stops at all. So I think it is a mechanical issue, not a brake fluid issue.

[tyang]

Hi John,

I second the M/C being the culprit. My daily driver had the same problem and the brakes got progressively worse when applying light pressure, but had no problem when I got hard on the brakes. A new M/C eliminated the problem.

Tom

[365gtc/4]

Thanks again Kerry and Tom. The guys over at the C4 website said the same thing. The explanation fitted my symptoms. So I will have to refurbish my MC. I will let you know what the exact issues are when I pull it apart next week.