I have a 68 442, power disc/drum setup, single piston calipers. My brake light has been on for a long time... I have replaced everything except for the front hard lines and calipers. Figured my proportioning valve was stuck.

Found these rebuild instructions on the forum: https://www.musclecarresearch.com/gm-3956727-rebuild.
Basically, you tap the brass line seats at either end of the valve, pull them out, clean the thing up, replace the guts, and press the new seats back in with a brake line plug. Pretty simple, pretty cool.

Rebuilt the valve, brake light is off and the valve seems to work. Freakin Sweet. But now my rear brakes won't bleed. Fronts bled fine. Bench bled the master, which did seem to have some air in it, but still no good. Everything bled fine before the valve rebuild. What am I missing!?

Two things:
I am concerned that the replacement brass seats in the kit seat further into the valve bore and are partially or totally blocking the ports to the rear line. What could I do to test for this?

If a freshly bench bled master cylinder were connected to an empty hard line, would is bleed correctly, or draw air back into the master?

 

Disconnect the hard line at the back of the combo valve. Does brake fluid come out? If so, the problem isn't the combo valve. Continue down to the back brakes. Could be a bad rubber line at the rear axle or rust plugging the bleed holes in the rear cylinders.

 

It would bleed correctly, just make sure you keep the master topped up with fluid after pumping the pedal up.

 

9 of 10 Xs its trapped air. Did you bench bleed the hell out of the master? Invest in a power bleeder. Best $80 Ive ever spent on an inexpensive pump style bleeder(not including all the adapters Ive purchased).

 

Thanks Joe, I'm going to re-bleed the master tonight or tomorrow, and try to gravity bleed as far as the rear line fitting on the proportioning valve and see if I'm getting good pressure there. If so, I'll work my way back to the wheels.

This seems like a no brainer in retrospect, I was working with speed bleeders at the wheels (I have taken them out and tried regular bleeders), and I was concerned that with almost totally empty lines I was just sucking air out of the lines into the master.

What would you guys recommend for a power bleeder? The girlfriend says theres no way she's helping me bleed brakes again... ever!

 

Motive pump style is a decent one. Make sure it comes with the right master cyc adapter. This tool will allow you to painlessly bleed your brakes. It should be done every few years for a car that sits, every 50-60K for a daily driver. Use fresh clean fluid of the right DOT rating. Never use fluid thats been siting around even if it looks ok.

Pump the pedal with the power bleeder on it to get the air out of the master.

http://www.jegs.com/p/Motive-Product...53543/10002/-1

 

Good news everyone!

Re-beled the master and gravity bled to the proportioning valve, got a healthy spurt of brake fluid there, so the problem isn't in the valve. With no helper and no energy all I managed tonight was a gravity bleed to the rear wheel cylinders, which was a bit anemic. Got some brake fluid on the floor, but the level in the master didn't drop much, maybe a quarter inch. Any insight there?

If I can find a helper over the weekend, I'll try again, otherwise I'll be waiting for that power bleeder to make its way to me in the mail...

 

Power bleeder (motive: 70 bucks from amazon with the adapter, ditched the stupid chains it came with and threw a c-clamp on there) did the trick. Pedal is firm and brake light is off!

Not sure exactly what the problem was, but I did get some trash out of one of the rear bleeders. Also, I think part of the problems was that I had overtightened and warped the speed bleeders I was using in the rears.