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lose brake pressure when engine running, wtf?

C

cjmahone66

CEG'er
Joined
Sep 24, 2008
Messages
116
Location
DFW, TX
SVT with 4 wheel disc brakes: i have a track day coming up in a couple weeks, so i decided to put on some new pads and flush the old fluid this weekend. now i'm having a problem and i need some advice please!

changed pads, that went fine, then i flushed fluids. started out with my mighty-vac, but couldn't get a good seal so i switched to the old fashioned method and had my wife help me. the fluid flowed pretty easy with the foot-pumping method and clean fluid came out with no bubbles. did all four wheels and the peddle felt nice and firm, i was real happy. didn't test drive that day because my washable air filter was still drying.
next day i go to take it for a spin, and the peddle is real soft, goes almost to the floor before it stops. when i turn the car off, the brakes are real firm again after a couple pumps. brakes appear to work fine when the car is off, then when it's running they go soft on me. what the heck did i do wrong?! obviously, i gotta get this figured out before track day.
 
Clutch release cylinder just borrows fluid from the brake master cylinder reservoir, don't see how that could contribute to a soft brake system, but anything's possible.

Maybe at some point while pedal bleeding, the brake MC went dry and you pulled some air into the system?

Have the rear brakes adjusted? Sometimes when you fully retract the rear pistons, they screw in too far for the new pad thickness and the system can't self-adjust. This could make for a long pedal, try self-adjusting the rear brakes by backing up slowly for a decent distance and applying the brakes multiple times. I usually preset the pad clearance by adjusting the rear caliper pistons a 1/4 turn at a time until there is too much drag, then I screw in the piston a 1/4 or 1/2 turn and let the system self-adjust after that.

Also, did you get the tang on the back of the inboard rear pads into the slot in the rear caliper pistons? If you didn't, the rear brakes won't self-adjust.

Sometimes when pedal bleeding the master cylinder gets stroked much further than normal, and the piston seals will wipe across a corroded section in the MC bore and tear the piston lip seals, now they don't seal and you'll get a long/soft pedal, only fix here is a new MC.

If anything else comes to mind, I'll post again.
 
crewchief- i did not bleed the throwout bearing. i don't expect it to make a difference, but it could probably use it anyway so i will do that just to make sure.

gm- those are some good suggestions. i watched the master cyl real closely during pedal bleeding, but i guess it is possible. i did have to use a c-clamp to compress the piston and fit the new thicker pads, could it be that i depressed them too far? when you say "screw in too far" do you mean that rotating the piston actually changes it's depth? they rotated a little when the c-clamp was pressing them inward. i did make sure that the tang was in the slot.
if the MC tore a piston lip seal, is there anyway to know for sure? or do you just deduce that by process of elimination?

you have both been very helpful already so far, so thank you.

edit: i think i am going to go pick up some new rotors too, i was trying not to spend too much, but since i have to get back in there i might as well replace the rotors. will new pads on old rotors cause problems?
 
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If you used a c-clamp on the rear caliper pistons, then they are probably ruined. The only way to retract the rear pistons is by ROTATING them. The front pistons you can use a c-clamp.


I would bet the long pedal stroke is because the threaded part of the rear piston is damaged because you forced them in with the c-clamp.
 
I just did the rear brakes on my '99 CSVT about a month ago, so it's fresh in my mind, the pads were worn almost to the backing plates (got my money's worth, I did...). I had to turn the rear pistons about 4 full turns to get them into the bores far enough to install new Mintex pads and new rotors. Agree with blu, if you re-seated the rear pistons with a c-clamp, you should look into some new/rebuilt rear calipers, because the self-adjusting mechanism is damaged. If the rear self-adjusters aren't working, you'll have a long pedal.

With new pads, either use new rotors or turn the old rotors as long as they're not near their minimum thickness after turning. Minimum thickness should be cast into the rotor near where the hat and the rotor substrate meet. Frankly, if you're open tracking the car, even just occasionally, new rotors is money well spent - running out of brakes on a road course can get expensive is a hurry.
 
F-me!
that's gotta be my problem, i ruined the calipers. SOB, i'm pretty angry at myself now. guess i can add calipers to the shopping list. thanks for your input, i'll report back when i get a chance to replace and confirm.
 
youd really have to crank a rear caliper piston to break it like that, while possible i would have expected you to noticed a terrible and noticeable noise when it finally broke under force...i'd be more expecting to see this car just need another bleed job!
 
problem solved

problem solved

problem solved. thanks for all your help!

i got lucky. when i took apart the rear calipers and tried to reconstruct what i had done, i realized that the c-clamp was actually turning the piston in, not pressing it straight in. so the rear calipers where fine. for good measure i sprung for a new set of rotors all around and when i was installing the front ones, what did i find?......

the spring clips were not installed correctly :eek:. fixed that and bam, everything works like a champ. now we should be ready for track day :cool:
 
Indeed, glad the rear calipers are OK. Those spring clips on the front calipers are evil incarnate, I'm pretty damn good at taking apart and re-assembling stuff, but I took a picture of those before pulling them off. I remember a thread about that quite awhile back. Right here... Am surprised I didn't remember that - too old, too much CRS.
 
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