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No rear brakes on 70 Challenger

Posted By: rickinpc

No rear brakes on 70 Challenger - 09/14/09 02:01 AM

I have disc brakes on my Challenger. The fluid will not go through the proportioning valve to the rear brake line so I have no rear brakes. The only place I can get fluid is the passenger front wheel. I can pump up the pedal and nothing releases pressure except the right front.

The master cylinder is remanufactured NAPA unit. The wheel cylinders are all rebuilt. New brake lines front to back. I have installed 3 different proportioning valve units and have the same problem. I

I disassembled two of the units and cleaned them out and thought I had it working but it still doesn't work.

I have the car on my lift and notice that fluid is dripping out of the back of the master cylinder when we are trying to bleed the brakes. I can't pull fluid down to the front brass block with my air-powered brake bleeder.

There seems to be a little fluid leaking from the brake switch, too.

I'm thinking it is the master cylinder. Any ideas? I've been building Mopars for 25 years and I have never had this problem before. Please feel free to recommend, critique, etc.. I'm stumped.

Rick
Posted By: 1_WILD_RT

Re: No rear brakes on 70 Challenger - 09/14/09 02:12 AM

Have you tried loosening the nuts holding the M/C to the booster? If the Booster pushrod is adjusted to long it will push the first piston into it's bore slightly blocking fluid flow to the rear brakes...
Posted By: rickinpc

Re: No rear brakes on 70 Challenger - 09/14/09 03:05 AM

The fluid pushes through the master cylinder to the top of the PV just fine. It shoots a great stream of fluid when open. When it is hooked up the plunger just stops like it is hard against something.

What do you think?

Rick
Posted By: 1_WILD_RT

Re: No rear brakes on 70 Challenger - 09/14/09 04:44 AM

Back in the 70's they sold a tool that would screw into the hole where the brake failure warning switch screws into the proportioning valve... It would lock the shuttle valve in the centered position allowing your to bleed the brakes... usually it isn't needed but that may be what you have going on... Basically there is a groove cut in the center of the shuttle valve so switch is open.. If pressuse is lost to either side of the shuttle valve the spool will shift... blocking the fluid path to stop the fluid from leaking out of what ever failure exsists on that side of the system... Also when the shuttle valve shifts it grounds the switch which would turn on the brake failure warning light.... You might try removing the switch, use a scribe to center the valve then try bleeding the brakes while locking the spool in the centered position...
Posted By: rickinpc

Re: No rear brakes on 70 Challenger - 09/14/09 07:49 AM

I took out the switch and cleaned it up. I depressed the shuttle valve with a small allen wrench and it moved freely. I tried to take the switch apart but the nut moves the whole center of the switch and doesn't want to come apart.
Posted By: 1_WILD_RT

Re: No rear brakes on 70 Challenger - 09/14/09 02:41 PM

The switch isn't the problem.. The plunger on the switch presses down on the shuttle spool in the valve... There is a groove cut into the spool that the tip of the switch plunger can sit into... Unless the shuttle spool is shifted to either side do to uneven brake pressure.. If/when the shuttle spool is shifted it blocks flluid flow to one side of the system... The theory being lower pressure means theres a leak so stop the fluid flow... However when you are trying to bleed the brakes it can block fluid flow for that operation as well... Usually it doesn't but yours may be...
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