Brake bleeding problem

Installed new 4-wheel disc system and hydraulics on my 72 Nova. Unable to bleed rear calipers properly. Have tried gravity, pressure, vacumn bleeding and combinations of those 3. Vacumn bled with 21" vacumn from vacumn pump. Have replaced proportioning valve and disassembled master cylinder to check internals. Bench tested master cylinder several times. Checked hydraulic line to rear calipers and blew out. Disconnected banjo bolts at calipers to check flow, very little. Car is level on jack stands. Have gone around several times trying to bleed from right rear, lr, rf, and lf. Front calipers are fine. Cannot bleed enough from rears to get any kind of brake peddle. Have tilted rear calipers to try to bleed from highest point. Master cylinder came chromed. Master cylinder problem? Have bled many brakes before but never this type of ongoing problem. Help!!

Is the proportioning valve for rear drums? In fact, should the proportioning valve remain when up-grading to rear disc?

i forget the name, but it may be the proportioning valve, is there a block on top of the rear axle which the rear brake line goes to? that may be failing/ failed. ifit is there, replace it, or remove it to see if it effects flow to calipers.

I think that proportioning valves are still needed to prevent rear wheel lockup and proper pressure to the front brakes during hard brakeing situations. I have actually tried 3 of them around this problem. Thanks for your reply.

That block was about the only thing which I hadn’t taken apart to check. Just got done doing so and is okay. Still only minimal hydraulic flow and very little brake peddle. Thanks for your reply.

IIRC that is called the distribution block. The proportioning valve is up at the master and controls the balance between the front/rear circuits.

I had real trouble bleeding the rear brakes on my Chevelle once. In a fit of frustration, I started pressing down much harder/faster on the pedal and finally blew the air out of it. Afterwards, it was apparent that the line made quite a high bend to clear the axle and it was a real bear to get the air pocket out of it. It is surprising that a pressure bleeder would have this kind of trouble though…

Bench tested master cylinder several times

I hope you meant bench bleed.

Do you have the rear calipers mounted correctly? Mount them so the bleed screw points upward.

Is the brake master cylinder designed for 4-wheel disc brakes, or for front disc, rear drum brakes?
When you mount the brake master cylinder, are you checking the push rod seating in the piston end of the brake master cylinder? Sometimes, the rod has to adjusted to the correct length.

Brake systems are engineered to perform on the car on which they were installed. Bolting on on a bunch of aftermarket parts and calling it a brake system is a good way to get yourself killed. The proportioning valve, master cylinder and rear brakes were all designed to work together…The stock parts that is. A master cylinder and proportioning valve for a 4-disc brake system will be COMPLETELY different than for a disc / drum or drum / drum system. It sounds like you have MAJOR parts incompatibility…Brake conversions like you describe must be carefully engineered to work properly…Fluid volumes and pressures must be balanced to achieve the desired results…