Okay I've had this problem for a week on my '89' The front brakes are doing all the work and the rears aren't really doing anything I already had all 4 wheels off the calipers are in good shape the brake lining on the pads and shoes are good the wheel cylinders are brand new The rears are adjusted properly no leaks anywhere. I had a friend hold the brake with the jeep on a lift and I can still spin the back wheels. When I bleed the back wheels nothing comes out. I disconnected the line going the the rear hose and still nothing. So I tried bleeding at the master and lots of brake fluid shoots out. Can this be a faulty proportioning valve

 

That would be my best guess based on what you described.

 

Could you have pinched the hard line?

 

Na I checked the lines I'm gonna go with the valve but thanks guys.

 

proportioning valve

it is ether bad or has tripped because of air in the back lines
not positive jeep has the safety valve but if you have a line failure
the proportioning valve will trip and cut of flow to that part of the system
so you still have bakes some times exes air will cause this

 

http://www.dippy.org/svcman/sm05d.html

 

I agree with the prop-valve diagnosis... I'm pretty sure they have the "shuttle valve" to block one circuit if the circuit fails.

 

How do I get it to untrip? if possible.

 

you need to fix whatever caused it to trip
look on the proportioning valve there my be a pin sticking out that can be pressed to reset
if not you need to open up the front bleeders use a hose to run down to a jar with a little brake fluid in it to help keep from getting air
then in the brake peddle one time kinda hard the pressure difference should get it to reset to neutral but if you do it to hard it will set to lock out the front

anew valve my be the best bet but make sure you fix any problems or the new one will trip

 

I'm having the same trouble and have tried everything on leaning towards the Proportion Valve did you get yours fixed if so how. Thanks alot!!