Problem with my '77 CJ7 brakes

[reynolds78787]

I recently bought a 1977 CJ7 and I have been having consistent problems with the brakes. After driving it for a while the front brakes started locking up. Originally I was able to fix this temporarily by bleeding the brake fluid and they would go back to normal. Eventually this became less and less effective. So far I have replaced the brake lines, the proportioning valve, calipers, and the master cylinder. I have actually bought three different master cylinders so far, and none have helped (there have been mix ups with the company ordering me the parts). I have tried shortening the rod that goes from the brake pedal to the MC and keeping it the length it came as, no dice either way. The MC I have now fits on the way the one that originally came with the CJ did, but it will not build any pressure in the back chamber. Front chamber works fine. I have tried bench bleeding the brakes and no fluid comes out (it's only pushing air, when it's pushing anything at all). Any ideas? This has been a problem for months now and I am slowly reaching my limit. Thanks for any advice y'all have.

 

[JR74CJ5]

As unlikely as it may seem, if while removing everything else out of the equation (bench bleeding / bleeding MC with lines running back into the reservoirs from the outputs where brake line would normally go) is not working, there's a problem with the MC still is my guess. Nothing else to be "wrong"...

I've seen and had bad MC off the parts house shelf, not consecutively as you're describing, but it is possible i suppose...

 

[IOPort51]

The last MC I bench bled required plugs rather than return tubes. One of the MC pistons will not prime until there is pressure on the other. I think the rear has to have pressure before the front will prime. Of course it being a Corvette disk disk MC may have had something to do with this.

 

[Lumpy Grits]

Replace the MC.
LG

 

[mtnwhlr]

It sounds like you are not bench bleeding the MC right before installing it.

 

[reynolds78787]

We found the problem!! My roommate finally figured it out. The CJ originally did not come with the hoses that go from the ports in the MC back into the reservoir, so we didn't know we needed them (definitely not brake experts here). Bought a pack of those today and installed them and...SUCCESS. Drove it around a little today to test it out and it seems to be working. Fingers crossed this solves the problem for good. Thank you for all your input!