Questions on Caliper Issues

Im having an odd issue of blowing out left rear brake caliper. My C2500 is swapped with an 03 2500hd rear axle with disc brakes and I have gone through 3 calipers now. Each time I will catch it leaking fluid which is caused by fluid being pushed past the pistons on the caliper. It appears there is too much pressure and it is blowing the seals out. Could this be caused by a bad ABS unit? Any advice could help

Did you install a proportioning valve out of a rear disc truck when you did the swap? Did you remove the hold off valve?

94burbk1500 said:

Did you install a proportioning valve out of a rear disc truck when you did the swap? Did you remove the hold off valve?

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It still has the factory proportioning valve that came with the truck. Do you recommend swapping in one from a disc/disc truck? And I’m not too sure what the hold off valve is to be completely honest

I'm not positive if these trucks have a hold off valve, they might not. I would definitely find a prop valve out of a rear disc truck, you're probably not sending the right pressure to the rear.

Lots of stuff has changed since I got out of the business. LOOK AT YOUR SERVICE MANUAL for both model years--that of the truck you drive, and the donor-vehicle.

Hold-off valve was used on disc/drum systems to prevent the front brakes from applying until the rear brakes had enough pressure to overcome the shoe return springs. The front brakes were "held off" until system pressure was established. Since rear discs have no return springs, the hold-off valve is not needed with a disc/disc system.

Proportioning valve affects pressure delivered to the rear brakes above a certain psi point. Below that point, no pressure modification is provided.

If you're blowing fluid past the piston of only one rear caliper, I'd want to know how the park brake is adjusted, how the caliper is mounted, and whether you have the correct width of rear rotor.

Yeah,, their is something going-on in that left side. If the pressure was high enough to blow-out 3 left side calipers, then why would it not blow-out the right side?? Just thinking,,.
In the disc/drum brakes their was a residual valve, which kept about 10lbs pressure in the rear wheel cylinders to prevent the cylinders from trying to suck air when the brakes were released,, that 10lbs kept the wheel cylinder cups expanded.
In a disc/disc system their is no need for a residual valve unless the master cylinder is below the level of the calipers,, and then they use a 2lb valve.
Actually, in the way the duel circuit master works, the pistons are not mechanically connected together, the front brake piston cannot exert much force to the discs until the rear piston has moved enough fluid to push the wheel cylinders out to start applying pressure to the rear drums.
With a disc/disc set-up you should have a disc/disc combination valve, (that's the proper name,) and let the ABS do the work of modulating the pressures in the lines.

Thank you guys for the info. It has rotors and pads less than a year old that match the OEM (at least that’s what I bought from the auto parts store) but I will measure them to make sure the width is correct. Is the hold-off valve mounted directly off of the backside of the ABS unit? I didn’t notice any valves from the firewall to the rear axle

On the new vehicles, the residual is in the ABS module if equipped with rear drum, but as stated, its only purpose to keep the cups inflated to keep air out. The proportion valve or hold-off valve is in the ABS hydraulic module, so the only think hanging outside is the pressure differential valve that turns the dash light on if a system pressure failure occurs. It does not effect any operation of the front to rear brake operation since it's mount before the ABS module. The hold off or proportioning is inside the ABS box.

If the piston is properly seated with a good square-cut o-ring, I can't believe that any amount of pressure with create a leak at the piston.

There has to be a left / right comparison to make sure the pads match, caliper moves freely and positioned correctly. Mount distance from a base point to the center of the rotor should be the same, The caliper mounting frame is the same & the same distance. Does this "Blow-Out" occur while the piston is still inside with full pad friction? Is the rotor the same?

The caliper that leaks with a good piston and seal & mount correctly is leaking past the seal, I would have to see photos of that and how much line pressure is there .

I would bet that 3000 PSI would not cause a fully seated piston to blow out a seal. Now, if the piston had pitting, scratches or something, then I would know why....but three? Wow!

Minimum rotor thickness is just before discard to keep the friction material if completely gone, will not fall out because the metal backing plate of the pad is too thick to drop out and fall on the ground. I still can't believe the seal/piston would leak. I just rebuilt thousands of these instead of pulling one out of a box. Wow! I got to hear how this one ends.
This is like knowing whats going to happen when Bugs Bunny sticks his finger, in the end of Elmers shot gun, then he pulls the trigger!