Interesting Brake Issue

[Newell33]

Hi Guys

I have an interesting situation on my '73 Spider. I've had a new master cylinder, brake calipers, hoses, and pads installed on the car for some time now. Once in a great while the break pedal will stiffen up (pedal throw shortens, and it acts like there isn't much vacuum assist) and the front brakes will actually stick on slightly. I first noticed this when I smelled the hot brake pads on the front of the car, and the car seemed to feel like it was struggling to move. I got the car home and jacked it up to spin the wheels. Both front wheels were very tight, and I almost couldn't turn them. The rears were not affected. I lowered the car back down the first time, jumped in the seat, started the car, and noticed the pedal felt normal again, and the brakes were unlocked.

The issue has since happened a couple more times, and each time I make sure there isn't a mechanical pedal linkage issue. Any thoughts on what could be happening? You would think that if the MC was sticking then I'd have more play in the pedal, and not less. Maybe I'm complely overlooking something here, but I thought I'd get everyone's opinion.

Thanks,
Josh

[TX82FIAT]

When it acts up is it after you have been driving for a while. It does not sound like brake fade and I'm assuming that moisture in the line would not be an issue since you have changed so many parts. I had something a little similar caused by the brake fluid reservoir not breathing or letting air in the top. The fluid level did not always adjust with the pedal motion because the system was not breating. I just had to replace the reservoir cap.

[Jocky]

It may be worthwhile checking the pushrod that lies between the servo and the master cylinder for the correct length. It may be too long or binding, which will cause the servo to operate prematurely.

[Newell33]

Thanks guys. I crawled under there the first time it happened and tried moving the pedal back and forth. I didn't see anything out of the ordinary with the pushrod, but I'll check again. I'll take a closer look at the caps to see if there's anything going on there.

Thanks,
Josh

[babu2001]

I had something similar on one of my previous car. I've had to heat a bolt near the brake lines, the rubber one especialy. I never hit it with naked flame but overheat and years of drying started the problem. I wasn't able to drive the car as usual. Allways pulling to left. I stopped and smelled the burned pad at the rear. I even burned myself by touching the left weel where I've worked with the torch My mechanic explained me that the heat simply accelerated the disintegration of the inside of the rubber line. It was now acting as a valve. Pressure force oil to pass the obstruction but to drain the oil back, no pressure was applied so the oil was still pushing on the pad and slowly returning to the master cylinder. So, even if you changed your rubber hose, it may have a problem inside. Even within layers of the hose making a bubble (like on the side of a tire) which explain why you're not seeing any particules. Best shot I'll do if I were you will be to change both hoses in front. And they are cheap so not too hard on the wallet. Hoping it will be as simple as that.

[So Cal Mark]

the next time it happens, try loosening the mc mounting bolts a couple of turns. If that frees the front brakes then the pushrod is misadjusted

[Newell33]

I've had a similiar situation on other cars where the rubber lines were collapsing, causing the same problem. All of the rubber lines on this car have been replaced. The pushrod is the obvious place to check, but it's odd that it doesn't do it all the time. I'll loosen the MC immediately after it happens next time and see if that identifies / fixes the problem.

Thanks again,
Josh