Brake question

[mod88s]

last lap of heat race had the rear brakes lock up coming out of the corner and literally came to a dead stop. got pushed to the infield, rear tires were locked up solid. sat while the next heat race came on, 5 minutes later, drove it in to the pits.. noticed that when they were locked up, the brake pedal was rock solid.... i put the laser on it and it wasnt that hot, i could put my hands on the rotors... what would cause that much pressure to lock them up like that???

[grt74]

sounds like a master cylinder problem

[mod88s]

How so? is there a way to bleed master cylinders? Is there a need to do so?? Ive never had this happen in 35 years of racing so Im kinda stumped. Is there a proper way to shim the calipers to the rotors?? I thought it was right but would like to hear how some of you do it or if anyone has any other ideas what would cause this

[rakracing]

are you running the wave type rotors in the rear

[RCJ]

I had a brake line collapse or kink on the inside.Acted like a one way valve.

[grt74]

If both rear brakes are locked up,I wouldn't think its a line,you did say that there wasn't any heat in the brakes on the rear,if that is so,I would look in the rear end or trans,if a brake was locking up i would think there would be a ton of heat there,we use hard (metal lines)so a line is not the problem,sounds like a rear end or trans problem if the engine is fine,rotor could have broke but i would think you would have seen it ?something jammed between the rotor and brake caliper maybe,or master cylinder is all a can think of,unless you have some kind of valve for the rear but I've never seen one but i would think there would be heat on the rotor then,but if it is the master cylinder you could just open the bleeder and see if there is alot of pressure there at the caliper,if there is alot of pressure there the master cylinder is more than likely bad because its not letting the fluid back,but there again i would think there would have been alot of heat at the rotors too,i have seen stuck calipers but there again i would have thought there would be alot of heat

[mod88s]

I Run scalloped rotors. Not sure if just one or both rears locked up. The calipers were warm about 125 degrees. the rotors were not very hot. You could smell the brake pads and see them flaking up on the edges. The brake pedal was rock hard when they were locked up. Not sure if its a caliper or master

[TheJet-09]

I'm with grt74, if there wasn't significant heat in the rear brakes I'd be looking at drive train. But for consideration, how cold was it where you race? Was the time spent in the infield along with the ambient air temp enough to cool everything down? But the lack of play in your pedal would lead me towards a brake issue. Play in the pedal obviously comes from the deflection/stretching of the caliper piston seals, which is what returns the pistons when you let off the brakes (so the pads don't drag too much). That, along with some inevitable runout of the rotors causes a gap between the pads and rotor, which equals play in the pedal. If that was gone, it'd tell me there was no gap (so either the calipers themselves or a MC). If it happens again, and they again push you to the infield, ask the tow truck to just pick the rear of your car up real quick. Even with a spool in the rearend, there's some rotational play side to side so you'd be able to tell what rear wheel was locked up and go from there.

[Matt49]

I'd be examining the brake pedal assembly. The fact that the brake pedal was stiff as a board points me that direction. Even if the rear master cylinder was messed up (locked up) or one of the rear calipers was locked up, the brake pedal should still move provided that the bias adjuster and front master cylinder are not also completely bound.
Check your bias adjuster for clearance of other bolts, chassis tabs, etc. throughout brake pedal travel. On some cars, the bias adjuster is known to have interference with one of the bolts that holds the pedal assembly to the chassis.

[mod88s]

It was stiff as a board as in you could tell it had a bunch of pressure built up, not locking up on anything. It moved a tiny smidge but not like normal. The transmission is fine, the rearend is fine, (checked everything) it is definately in the brakes at some point. The pads are almost new, each pad about 1/2" thick or so. The air temp at the time was in the low 70's. Sat in the infield for about 5-10 minutes before i was pushed off the track,, i started the motor and drove it back to my pit. So if the brake pads are dragging a bit, lets say there is a high point on one of the pads, wouldn't that wear it down so it mates with the rotor? i can see it building a little heat, but I think it would wear flat and mate with the rotor, maybe not. Would old fluid (maybe moisture in fluid) cause this issue maybe?

[TheJet-09]

Is the clearance not right side to side on the calipers then? Meaning there isn't enough room for the pistons on one caliper or one side of a caliper to push back in some as everything heats up and expands? You could push the pistons all the way in on the calipers (flush with the body of the caliper) and stick the pad in. When mounted you should be able to at least wiggle the pad a little. The pads will wear through time, but there could also be an initial point of no return if there was virtually zero clearance (meaning everything would heat up/lock up before wearing to a proper mating surface). Lastly, I would believe the opposite effect would occur with water in your fluid. Water has a much lower boiling point than brake fluid which would/should cause a poor pedal or inconsistent.

[Kromulous]

Bias adjuster i bet, or something in pedals.

[mod88s]

Is there a proper process for shimming the calipers? I have some caliper shims, and i know there are some shims in it now.

[TheJet-09]

I just use washers between the caliper mount and the caliper itself, usually about two thin ones per bolt. I set the caliper over the rotor and snug it up to the mount, then check the clearance between the rotor and the caliper body (where it hangs over the rotor). I try to get that as close to equal on both sides as possible.

[rakracing]

my feeling is something jammed between rotor and caliper and twisted, did you try to back up first before you drove it to the pits, check the calipers on the bottom side for marks