Brake lockup

[Cobra Dude]

Arrrggghhh!!! Recently got my car back and it just developed a new problem where the brakes are locking up after a bit of driving. I know it's the brakes since the car drives with significant resistance, and there is a strong smell of brakepads.

This morning, with the car cold, I jacked up all wheels independently and was able to rotate them freely by hand. Then I went out for a 2-mile round trip and did the tested again. Every wheel is locked tightly enough where I cannot rotate them by hand, and cannot push the car in the driveway.

Now, since it's all 4 wheels, I have to assume that the problem is either related to either the vaccum/booster system, or the fluid/master-cylinder system.

Any idea how to test and narrow this down from here?

Thanks,
-Neil.

[jwheel55]

Possible master cylinder recuperating valve problem. Pressure built up when you apply the brakes is not bleeding down when you release the brake pedal. Open a bleed valve on one caliper and see if the fluid streams out under pressure. If it does it proves that the master cylinder is at fault. Also check that there is clearance between the push rod and piston when the brake is fully off.Some systems are adjustable. You can rebuild mastercylinders with new seals but this is a safety critical item so a new one is probably the best way to go. The vacuum diaphram doesn't usually cause this problem . Generally if it fails it reduces the power assistance making the pedal heavy in use. Hope you find this helpful , jonny.

[Cobra Dude]

This sounds great if it is the problem, only because I have another master cylinder laying here that I was supposed to have swapped in a long time ago (when I converted to 4-wheel discs).

There was another thought that water/moisture in the brake lines could cause this effect since it boils at a lower temp and expands in the brake lines, and a system flush would take care of that, but that will happen if I swap in the MC as well. I'll go experiment this weekend.

Thanks,
-Neil.

[jwheel55]

Water in the brake fluid is caused by the fluid being hydroscopic which means it attracts moisture from the atmosphere. This has the affect of lowering the boiling point of the fluid which under long heavy braking which heats the calipers causing the the moisture in the brake fluid to vapourise. This just makes the brake pedal sink further to the floor because the vapour being air is compressable. This isn't your problem I would say. Conventional brake fluids should be changed regularly to avoid this problem and the internal rusting of the system that can occur. I use a silicon based fluid in my car which stops the rusting and avoids the vapour problem as it doesn't attract moisture. I'm not sure it is advisable to use it under racing conditions but I'm sure some of our racing friends will advise. Jonny.

[Cal Metal]

jwheel:

I spoke with the legendary Bob McKee about his exact issue a couple of weeks ago. One of the sister Echidnas was doing some testing at Lake Geneva. McKee was there doing the consulting and engineering. I know the positives of Silicon fluid and asked him their applicability to racing. He said that, "generally no.....it has a tendency to compress, particularly if all the air is not flushed from the fluid". He said that they do like to use it in restoration work, though, as it lacks the hydroscopic quality of DOT 3 and saves wheel and slave cylinders from pitting.

[XXQQQME]

I had the same problem with my Cobra. It seemed that vacuum assist was actually sucking the brake pedal down. I swapped out the relief valve which didn't solve the problem. I also jacked the car up and found that I couldn't spin the wheels. I discovered, by mistake, that if I manually pulled up on the pedal with my foot, the brakes would release. I tried a stronger pedal return spring and it seemed to help, but the setup was Mickey Mouse.

I hope it makes you feel better to learn that I toasted my clutch trying to get the car rolling, not knowing that it was a brake problem all along. After having the clutch rebuilt and the flywheel refaced, I managed to blow a hydraulic hose to the throwout bearing. I've been towed home twice!

Anyway, I removed all the "old" $hit and replaced the pedals and cylinders with Wilwood racing components. I had to sacrifice my power brakes, but 11 inch rotors all the way around does the job quite sufficiently. Wilwood makes quality racing "stuff" and modification for installation was minimal.

Good Luck!

P.S.: that smell may also be your clutch, be careful!!!

[wadesdad]

I vote for pulling on the brake pedal to see if it allows the fluid back into the resevoir. If it doesn`t help try the bleeder valve, but watch out I`ve seen them shoot to really impressive heights.