baffling braking problem

[jackw]

I am hoping that someone can give me some insight into this problem. I have an 82 spider. When it it sitting in my garage and not running and I step on the brakes, the peddle is right at the top and feels great. But when I am driving the car, sometimes the pedals feels fine, but then all of sudden the pedal will almost go to the floor. I still have brakes but I know the pedal shouldn't be this low. Has anyone ever had this problem and could it be the booster going or a vacuum leak? Thank you for any input.

[spidernut]

I'd recommend bleeding your brake system. It sounds like you have air in the brake lines. The old-fashioned method or a brake bleeder should do the trick. Your brake booster and/or vacuum leaks would cause a hard to press pedal versus a pedal that goes to the floor.

[maytag]

I'd recommend bleeding your brake system. It sounds like you have air in the brake lines. The old-fashioned method or a brake bleeder should do the trick. Your brake booster and/or vacuum leaks would cause a hard to press pedal versus a pedal that goes to the floor.

I'm with spidernut:
don't bleed your brakes because it has anything to do with this: do it because it's good preventative maintenance. I don't think this sounds like air in the system, but maybe moisture (brake fluid is extremely hygroscopic, and will absorb moisture right out of the air. Even in my dry climate, I still change the fluid annually)

But this problem specifically sounds like booster-related.
I'd recommend replacing the check-valve and the hose with new. Autoricambi lists both on their site, inexpensively.
Start there, and then see what you get.

[So Cal Mark]

the check valve or vacuum hose won't cause a low pedal. Warped rotors or excessive run-out in the rotors will cause "knock back" of the pads. When this happens it takes more pedal travel to push the pads out to the rotors

[4uall]

the check valve or vacuum hose won't cause a low pedal. Warped rotors or excessive run-out in the rotors will cause "knock back" of the pads. When this happens it takes more pedal travel to push the pads out to the rotors

based on the above, once I replaced my rotors & pads would the pedal adjust itself accordingly?

[maytag]

the check valve or vacuum hose won't cause a low pedal.

i agree... but they can cause a HIGH pedal, which is being assumed is the normal mode. but could it be that what he is thinking is the fault mode (low pedal) is actually when the booster is working, and he needs new pads??

I still say that $12 in parts for a check valve and new booster hose is a cheap way to make sure you're chasing the right problem on a 35 yr old car.

[FiatMac]

... but could it be that what he is thinking is the fault mode (low pedal) is actually when the booster is working, and he needs new pads??

Worn pads don't affect the pedal height. The hydraulic system compensates for pad wear by pulling brake fluid from the reservoir. Think about what happens when you put in new pads. You back the piston out until you get enough clearance to install the caliper over the new pads. Then you pump the brake pedal, pulling fluid from the reservoir, to move the piston back out to the pad. The same thing happens as the pad wears.

[azruss]

my thought is you have a MC that is starting to fail.

[81SPIDERMATT]

I'd recommend bleeding your brake system. It sounds like you have air in the brake lines. The old-fashioned method or a brake bleeder should do the trick. Your brake booster and/or vacuum leaks would cause a hard to press pedal versus a pedal that goes to the floor.

this one first... cheap and easy and could be the problem.... take out the cheap and easy first

[djape1977]

master brake cyl is about to croak

[jackw]

I just finished bleeding the front brakes on my spider. Tried to bleed the back but even though I had it up on jackstands I could not get them to bleed. I read a post form someone that say he disconnects the compensator link to bleed his. If bleeding the front does not work, I will probably try this but not today. I live just south of Raleigh in North Carolina and today in my garage it was 85 degrees with 93% humidity. There"s supposed to be a cold front coming in so I will wait till then. Will keep you posted. Thanks all for the replies.

[deftone]

I live just south of Raleigh in North Carolina and today in my garage it was 85 degrees with 93% humidity. T.

Thats a winters night here in Houston

[majicwrench]

Bleeding is not going to improve anything. If it had air in system would have low spongy pedal every time. Same with pad knockback, you would always have a low pedal.
LIke has been posted, you have a master cylinder issue, pressurized fluid is leaking past the cup.
Keith

[katsi]

I have the exact same problem. The brakes seem perfect untill you start the car and then it goes soft. I have a new MC, rotors, and brake pads. I have not been able to solve the mystery. I am almost quite sure that it has something to do with the booster. Maybe, I am getting too much assistance from the booster due to the fact that I have swapped out the intake and carb for and earlier version. Prior to that, I did not have the problem. Maybe I will try to cut off some of the vaccuum pressure from the hose and see if that helps. Good luck with yours. I assume your brakes work fine as mine do. Just the lower end of the peddle - right?

[majicwrench]

You 've a diff problem, the key word in the OP is "sometimes" if your pedal goes low every time engine started you've likely got air in system. Power assist is simply helping your compress it.
Keith