My vehicle is a 72 monte carlo with power brakes, disc in front drum in back all stock setup. My front brakes work fine but my rear brakes don't work, if I jack the back tires off the ground and put it in drive so they start spinning then hit the brakes and keep the petal down what happens it the rear tires stop spinning for a second and then start spinning again just from the compression. I blead the brakes and they didn't have any air in the lines. I was just wondering if there was some sort of valve that would prevent loosing all braking in the even that a line broke, which may have gone bad? I've checked for leaks in the rear brake lines and haven't found even a hint of leaking fluid. Any info or suggestions would be greatly appreciatted, thanks!

I had a similar problem on a couple cars. When you bleed the rear brakes, you apparently get fluid flow and no air, but does the fluid actually squirt out when the pedal is being pressed? Mine did not. The fluid would come out but not with any real purpose and the pedal would always go to the floor when pressed after bleeding. Both times this happened (on my Chevelle and on an 83 Grand Prix) it turned out to be a stuck proportioning valve. Not saying this IS your problem, but it happened to me - twice. For the Chevelle I got an aftermarket one from OPGI and it worked like a charm.

The proportioning valve should be bolted to the left frame rail right under the master cylinder. It's a beach to do, especially if you have headers, and you'll likely be replacing some brake lines with it, but it was worth the work for me.

Yea I think that would probably be a good place to start, any suggestions on what I should replace the proportioning valve with?

are the shoes properly adjusted? if they are way loose, they won't travel far enough to make contact.

Well, like I said above, I got mine from OPGI. It was an aftermarket one, so I didn't look like the original which was made from a cast iron housing. This one appeared to be machined from brass so it was all square-shaped. The mounting holes were right on the money and the electrical connector was the correct type. I seem to remember it costing about 50-60 bucks. That's a couple years ago.

Check the brake pedal pushrod...if it 's too tight it won't allow the M/C piston to return fully. The piston will cover the rear port which won't allow fluid into that chamber.

are the shoes properly adjusted? if they are way loose, they won't travel far enough to make contact.

X2.
There should be a slot in the inspection cover. Slide a wide flat screw driver in and you will feel the gear. With the tire off, Rotate the adjuster gear as you turn the drum. When you feel a drag on the drum it is adjusted...

yeah this weekend when I'm home I'll check the pushrod and I'd like to order an adjustable proportioning valve but I'm not sure how many inlets/outlets it needs to have, I'm assuming its got 2 inlets and 2 outlets does anyone know for sure?

An adjustable prop valve should only need one inlet and one outlet. I goes in the main brake line that runs along the frame, feeding the rear brakes.
At least that's how mine is...

So the proportioning valves that summit sells with just a single inlet and outlet simply restrict the flow to the rear brakes? If this is the case do I still need and origional equipment proportioning valve and the the one from summit would just get installed anywhere after the original valve and the rear brakes?

If your original combo valve is still in place...that's all you'll need for front disc/rear drum brakes.
Nothing else.

When you bled the brakes. How much pressure did you have coming out the rear bleeder? A good stream or was it weak like a trickle amount of fluid?

not to hijack this thread but i am having a similar prob. i am getting flow to the rear but just a little poof, nothing substantial at rear bleeders. just did a conversion from drum drum to F disc/drum. put a combo valve in where fact. junction block was. i dont think the valve is stuck. the idiot light on dash tested fine, working correctly. and not on. did i get a bad MC from the brake supply place? fronts are fine, nothing to rears. looks like rear brake line was spliced sometime ago. i dont think wheel cyl. are stuck. help!