1974 Rear Brakes Won't Get Pressure
 

1974 Corvette has not had rear brakes for some time. Pads had no wear. So far we have changed:

1.) M/C as it was leaking out back
2.) Left caliper (RH was rebuilt)
3.) All lines and fittings from prop valve back

We have bled many many times. Sometimes there is a stream, other times it is anemic.

We have notice the proportioning valve is ALUMINUM. All the ones I see are brass leading me to believe this had been changed. We are getting power to the wire that controls the light on the valve but the light is not being tripped.

Any suggestions?

Did you bench bleed the MC?

After bleeding the Master Cylinder, you probably need one of these, https://willcoxcorvette.com/corvette...ng-valve-74-77 and also one of these https://westernchassisinc.com/Propor...ve-Bleed-Tool/

The brown wire on the valve connects to ground to light the bulb on the dash.
With the key ON disconnect the wire and touch it to ground, the dash light should be on.
If not pull up the parking brake and see if the bulb lights up.
If not then the bulb could be bad or a problem with the wiring.

General information:
Under normal braking the BRAKE light will never come on, it only comes ON if there is a leak or some other failure in the brake system.
Some valves will go ON and stay ON, others go on and off as the pedal is used.

Yes, several times. Thank You.

There is a sliding piston inside your distribution block that moves according to differential pressure to block off one side or the other. If it has moved all the way over to block off the rears, it should be providing ground to the light to close the circuit in the dash. You might check to see if the post on the distribution block connects to ground which would be a sure sign that that piston has moved over. You've mentioned that your dash light doesn't light so it's probably not grounded but that is easy enough to check.

I didn't know there were aluminum versions of this block. Later C3 versions have an proportioning valve. If you have that that also could be limiting the flow to the rears. Here is a cutaway of that later valve.

There's been lots of discussion about this particular "feature" and I don't really remember or want to look up when the changeover from a simpler block to the one with the added proportioning valve kicked in (79 maybe)but it is worth a further look on your part to determine what you have.

https://cimg3.ibsrv.net/gimg/www.cor...215d06015.jpeg

Here is my post on that subject, hope something here helps you get to the problem's solution

https://www.corvetteforum.com/forums...ing-valve.html

I think you should have a block that looks like the one I pictured.

Have the rear brake line hoses ever been replaced?

rear hoses
 

Yes, rear hoses, brass fittings, and all metal lines from prop. Valve back. I have reasonably determined that the aluminum prop. Valve is from a 79 to 82 as the 74 to 77 valve should be brass. I've replaced everything else, so I will complete with a new valve.

What the Problem Was
 

It turned out to be the proportioning valve. It was sucking air at the switch port.