Internal Brake Pressure Loss?

[Chris91GT]

I've been developing a problem with the brakes on the Stang recently. The symptoms are quite simple... the pedal is rock hard initially, but after a long straight, or several corners with minimal braking the pedal pressure is soft and travel is significant. If I tap the pedal a couple of times or drag is lightly with the left foot approaching a braking zone the pedal is solid again. Once under pressure there is no pressure loss.

My guess is that I am losing pressure slowly within the master cylinder... is it possible to slowly bleed fluid from one side of the piston to the other when no pressure is applied to the pedal? If so, I suspect that a rebuild/reseal would solve this problem pretty easily.

[haniforama]

What about worn wheel bearings? The rotors might be allowed to runnout from true and push the brake pads away from the rotor causing the initial spongy feel...

[Chris91GT]

I just had the front bearings repacked and one of the outers was worn and replaced.

[Marsh]

Originally posted by Chris91GT
I've been developing a problem with the brakes on the Stang recently. The symptoms are quite simple... the pedal is rock hard initially, but after a long straight, or several corners with minimal braking the pedal pressure is soft and travel is significant. If I tap the pedal a couple of times or drag is lightly with the left foot approaching a braking zone the pedal is solid again. Once under pressure there is no pressure loss.

My guess is that I am losing pressure slowly within the master cylinder... is it possible to slowly bleed fluid from one side of the piston to the other when no pressure is applied to the pedal? If so, I suspect that a rebuild/reseal would solve this problem pretty easily.

Sounds like my brakes. I have heat issues, pure and simple.

[Greg F]

Chris, what was the cause of the problem?

[Marsh]

OK I just re-read this and it occurs to me that your brakes sound a little like the problem we had on the Western SAE car a couple years ago. We went through 3 bad master cylinders. They would hold pressure if you got on it hard, but light pressure would result in fluid leaking past the piston and the peddal would just sink.

You wouldn't be running Willwood would you? If so take them off and melt them down for scrap. Willwood products should never be used for anything other than door stops IMO.

[miataboi]

Originally posted by Marsh
Sounds like my brakes. I have heat issues, pure and simple.

Are you sure they're not weight issues?
;) :p :D
:eek:

[wpfri]

Marsh you are right on.Willwood.Need I say more!

William

[GR8 Ride]

Chris,


A couple simple things you can do to isolate where the problem might be.

With the car off (ie, no power booster), hold down the pedal. If the pedal stays high and hard, then it's not your MC that's leaking. If it slowly sinks to the bottom with pressure, then your MC is likely bad.

The symptoms sound distinctly like boiling fluid, which I'm assuming you've checked for.

The only other, bizarre thing I could think of is that you're getting too much boost from the brake booster; possible vacuum problems?


Pat

[P.Po]

Chris,

To me this really sounds like your your brake piston on some caliper is retracting while you are going over bumps and cornering. Usually a wheel bearing issue.

Can you can repeatedly use the brakes after short bursts of acceleration, and they are hard, and don't fade?
Do you drive for a bit without using your brakes, then they take a push or pulse before they harden up? If these are true then I really believe that a loose wheel bearing is knocking your piston back into the caliper.

Actually, now that a re-read the thread I am just saying exactly what Haniforama has said. He just has a much easier way of saying it.

Hope this helps...

-Patrick

(Edited because I suck at grammer)

[P.Po]

Chris,

What about the rear wheel bearings?

-P.Po

[CobraStang]

Guys, note that Chris' posts are almost a year old! The way he's been haulin ass this year, methinks he's got the problem sorted out!

[Greg F]

Chris originally posted this thread in August 2003. I should have mentioned this before I brought it up again. :p

I hope his brakes have been fixed by now and he can say what the cause was.

[Marsh]

There's nothing wrong with keeping tech discussions going, because someone might search the same problem and find this post. I remembered it from last year, but never really thought that much about it. When Greg responded I read it again and thought of the master cylinder.

[Chris91GT]

Well, now that you guys are bringing this up again... here are some issues. Yes, I do still have some sort of brake problem.

1) I rebuilt the master cylinder and that seemed to help quite a bit.

2) As P.Po. stated below, that's one problem I still have. This tends to be a problem on the big tracks like Mosport. On the DDT, Shannonville, TMP, etc I never have brake pressure problems. But at Mosport and Watkins Glen I need to tap the pedal before braking after long straights. This still seems to be a problem in the limited laps I put on the car at Mosport back in June. FYI, the front wheel bearings were repacked last fall.

3) Yes, Wilwood brakes. I have definitely noticed some massive flex in the front calipers.

4) PF pads are crap... Bill... need help!!!

[Greg F]

I inspected my master cylinder on the weekend, but the inner piston wouldn't come out (and there were no screws holding it in.)

I'm getting a new master cylinder installed tomorrow, and hopefully that will cure my problem.

[sjagman]

chris,

It sounds like the pistons are being pushed back in sligthly. On Jaguars this is typically due to wheel bearing endplay and front stub axle flex. I have had this problem.

The way I have cured mine is to adjust the front wheel bearings using the Porshe factory procedure vs. the Jaguar.

The porshe method is to basically remove the endplay to zero or as near to as possible with NO extra drag when rotating the wheel.

In other words keep adjusting it out and rotate the tire, when drag is notcie( i.e. the wheel dosen't spin as long) then back off just a hair. and tha is it! It REALLY made a difference with my car.

Hope this helps
Steve Kee
1992 XJS