Brake bleeding and red brake light

[Orence1]

Red brake light came on when driving home from the body shop a few weeks ago on my 72. Found the master cylinder rear reservoir completely dry. Filled it up, started the engine and the light was out. Came back two days later and light came on again. Went ahead and bled the rear brakes today using a vacuum pump brake bleeder. I ran about 4 oz. through the left cylinder and about 8 oz through the right. Made sure the master cylinder did not go dry. Did not get a clear stream but directions indicate small bubbles caused by connections not being air tight can occur. That seemed to be the case. Red light still is on. I took a test drive around the block and I have a right rear brake that is locking up with moderate pedal pressure. Any suggestions before I let my mechanic loose on it?

[65standard]

You need to find and fix the leak first.

[REEDS 72]

You need to bench bleed the MC if one of the reservoir went dry then it needs to be done make sure there are zero leaks of any kind hopes this helps.

[hamjet]

Sounds like the wheel cylinder is leaking on the brake drum,( the one locking up) hense the fluid loss. As far as the light, it sounds like metering valve pin on the end of the combination valve has to be held in while adjusting the front disc brakes. that sets the light. I think the rear res goes to the front brakes..

[Orence1]

Thanks for the help.

[mechanicalman]

Originally Posted by Orence1

Red brake light came on when driving home from the body shop a few weeks ago on my 72. Found the master cylinder rear reservoir completely dry. Filled it up, started the engine and the light was out. Came back two days later and light came on again. Went ahead and bled the rear brakes today using a vacuum pump brake bleeder. I ran about 4 oz. through the left cylinder and about 8 oz through the right. Made sure the master cylinder did not go dry. Did not get a clear stream but directions indicate small bubbles caused by connections not being air tight can occur. That seemed to be the case. Red light still is on. I took a test drive around the block and I have a right rear brake that is locking up with moderate pedal pressure. Any suggestions before I let my mechanic loose on it?

When brake fluid gets on brake shoes, they start locking up; I'd suggest checking the RR brake cylinder. Everything you say points to the R side as you pumped fluid through it and still saw bubbles (sucking air into the fluid from the cylinder). If the shoes are wet, replace both rear brake cylinders and shoes. Surprised you didn't see a fluid leak or traces of it somewhere. Has it been raining there? Water washes that stuff away in a hurry.

Then vacuum bleed it again and you should be OK.

[Stocker]

Originally Posted by mechanicalman

Everything you say points to the R side as you pumped fluid through it and still saw bubbles (sucking air into the fluid from the cylinder). If the shoes are wet, replace both rear brake cylinders and shoes. Surprised you didn't see a fluid leak or traces of it somewhere.

Agreed. I went through this last year, kept getting bubbles at the rear wheel cylinders. Turned out they were both bad but such a small amount of leakage that I never noticed it. New wheel cylinders did the trick.

[Orence1]

Again, many thanks for taking the time to respond.