Gentlmen: Years ago I had a similar priblem on my 69 Cutlass. After money spent and much vexing thought, I had to replace the little 90 degree check valve when it fell apart. All the problem went away. I since learned to start with the little things first. This is the plastic fitting on the power booster that is vacume fed from the engine. I hope this helps, Mike
Starting with the simple things is always a good plan for diagnosing a problem, but in this case the plastic check valve on the booster won't cause the "BRAKE" light on the dash to come on. Only a hydraulic problem (or a sticking e-brake pedal) will cause that.
Interesting as I have a 73 Toro (disc in front, drum in back) and similar problems. Brakes don't work but it happens after I have been driving a while (20 minutes or longer) and my brakes fade away. Kind of like the rear only is working. (Good thing the 455 engine is there as I down shift quickly to slow down!)
I changed the booster and master cylinder and pedal bleed the brakes. Now the light is on and the problems is the same.
I susepcted the proportional valve but wanted to find out more about it. I believe this year the prop valve and distribution are in the same housing.
I plan on bleeding them again this weekend but am interested in the button on the prop valve.
You seem to have a good knowledge on these any suggestions?
Yes, your 73 will have the combo valve that incorporates the proportioning valve, the residual pressure valve, and the differential pressure switch all into one housing. If you pedal bled the brakes and the light stays on, your differential pressure switch spool is offset to one side in the housing. If sludge builds up in the housing this can cause the spool to stick, which keeps the light on and can also upset the flow of hydraulic fluid. Depending on which way the spool is stuck, cracking the bleeder screws on the other end of the system can help recenter the spool. Since your rear brakes seem to work, try bleeding the fronts to get the spool recentered.
Yes, you do need to keep the button depressed during bleeding. This actually disables the residual pressure valve during bleeding.