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Binding Brakes - any suggestions?
I have Baer brakes on the front of a newly built Cobra, with 13" rotors, and they stop the car just fine...BUT TOO FINE.
After driving the car for a couple of miles, the front brakes just seize up solid, REALLY bad. It gets worse the more you drive them...horrible smell coming from them, and the minute you take your foot off the accelerator, they cause the car to just stop instantly...'very disconcerting' is an understatement. Jack up the front end and the wheels are completely locked. Leave them to cool down, and the next day, the wheels rotate perfectly again. Anyone have ANY idea what might be causing this? Glyn |
Do you have power brakes?
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Glyn,
I believe that the push rod from your brake pedal is too long. If the rod is adjustable, then the rod needs to be shortened. This keeps the piston, and sealing cups in the master cylinder blocking the return hold. While you drive the car the brake heat up.The expanding fluid is returned to the master cylinder. If this hole is blocked the brake will apply on their on while you are driving. Some time to the point the car can't be moved at all. A quick test for this would be after the brake lock up,to crack the brake lines where they come out of the master cylinder to release any build up pressure. If the car becomes drivable after this, the push rod is too long, and blocking the return port in the master cylinder. The adjustment may be only as little as 1/8 to 1/4 inch. How do I know? I have been there. Good luck Don |
A cleaner way to check the push rod is to drive the car until you feel the brake problem. Then jack it up. Then with the wheels stuck, loosen the nuts holding the master cyl. to the booster or bracket/firewall. If the wheels turn free the problem is most likely the push rod length.
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Do you have any free play in the pedal before it starts to brake? Does the freeplay decrease with use and temperature? Has someone plumbed a check valve into the front by mistake? Are the pedals set up to return freely to an off position when the pedal is released?
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Mike...non power assisted brakes, just regular old leg power.
Mickmate...I will check the free play, although I don't think there is any as the rod is directly connected to the pedal, so physics would dictate it starts to compress as soon as I press the pedal. No check valve as I built this thing from scratch, and didn't build one in. The pedals are quite free to return when released, although there is no 'spring' attached. Cobred...nice tip. I shall try this tomorrow. Glyn |
If the rubber lines going to the calipers are old they will swell. The fluid can get to the pistons because of the pressure the power brake unit supplies. The problem is the fluid cannot return because the swelling overpowers the return pressure. This happened to me on my street car. the hoses were the last thing I changed, and the cheapest.
It was always worse following hard use of the brakes. Two hours later they were fine,. Trust me! Bob |
Bob...good suggestion, but this is a brand new car, and the front brake lines are braided steel not rubber.
Glyn |
Other than the pushrod adjustment mentioned, I would check the return orifice in the master cylinder. Although it was a brand new build, after painting a '70 Mach I I had built about 15 years ago, the brakes had similar issues as yours. All the hydraulics were new other than the time sitting while I prepped and painted the car. Some point in that period, whatever moisture was absorbed from the air settled into the small return orifice within the master cylinder...after a few applications it would gradually get stiffer and then bind. After setting a while they would be O.K. until the next "drive". Although it wasn't completely obstructed, that's why it would release after setting a while, it slowed the return enough that the "drag" heated the fluid up to the point of brake lock. I also saw a similar thing quite often in motorcycles that would set over the winter, I think it was caused by a combination of moisture and the detrioration of the aluminum master enough to allow the contaminants to block or plug the return passage.
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There are a number of suggestions already posted, I will not add to those.
May I suggest: A positive return location with a spring. This will allow the pedal to start in the same location each time. Check the reservoir to reservoir transfer. If one is too large, you pedal travel has to compensate and you may exceed the recommended travel for the master cylinder. Most disc brake systems use the "square cut seal" method to retract the piston face back into the caliper. It has been used for several years successfully. If the brakes are not retracting, either the seal has lost its ability to retract the piston or there is a tolerance problem ( the corrosion changes the tolerance of the piston ). Corrosion on the piston is the number one cause of a failure to retract the piston. You may want to have the calipers completely disassembled and checked. Hey, these are just suggestions. :D |
do you have a line lock? a malfunctioning line lock will not allow the pressure to be released.
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Guys...GREAT suggestions, and many, many thanks...I will research and (please god!!!) fix it and then post what was the cause. Woodz...how did you 'check' and or 'clear' the return orifice?
Glyn |
No one mentioned this but is it possible the brake line is too close to a header thus boiling brake fluid? I don't know if the effects would be the same but you say after it sits for awhile it works fine, so pressure is being relieved some how but caused by what. Good luck!
Seems like you have a lot of brake for such a small car. It must just take a toe tap to stop. John |
Thanks to everyone who took the time to respond to my 'amateur' question. I LOVE this forum for a) all the help everyone gives on what are often obvious things to the more experienced builder b) all the grumpy old f&rts who are on it (like me!!) with an opinion about EVERYTHING c) the true feeling of 'family' in everyone who owns (or wants to own) one of these magnificent cars.
I shortened the push rod by a mere 1/8" today, and it appears to have corrected the problem. I drove it round our track quite a few times today, brought the engine up to operating temperatures and used the brakes a LOT to test it, and by the end of the run, it was coasting along with no front wheel binding at all. Longer drive tomorrow to give it a real test, but once again, it appears that Club Cobra Forum contributors really DO have all the answers! Thanks folks Glyn |
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