It doesn't limit flow if a line breaks, that's not what I meant.

Part of it is a proportioning valve. It limits fluid so the brakes don't lock up, that's why "combination valve." It's a pressure differential switch and proportioning valve.

Maybe the proportioning part has failed:

PROPORTIONING VALVE
The proportioning valve is used to balance front rear brake action at high decelerations. The valve allows normal fluid flow during moderate braking.

The valve only controls fluid flow during high decelerations brake stops.

 

yes I see that would cause the spongey pedal on steady preassure and a solid pedal by slamming on the breaks learn something new everyday here

 

My thoughts are both rear brake cylinders are toast remove both sides at the same time get to the point where all you have is the brake lines sticking out then try and get fluid to come out of them with some forceful peddle pumping and see what comes out if anything as you say.

From what you are saying you are getting nothing going to the back brakes correct?

If so this would be a good simple test to see if you are correct and see if anything does come out like some kind of clog.

My back brakes were so mucked up last year as to all my brake fluid in the back was like a thick black muck. Freaked me out that it could be that bad what happened was the cylinder seals had decomposed and leached into the break lines into a thick black muck.

What really freak me out was I probably had no back brakes for at least one year or more

With this test you will get an answer a bit of work but not all too bad or hard

One suggestion get two large bottles of DOT 3