The Stovebolt Site Forums The Shop Area Driveline Bleeding Brakes (anyone ever had this to happen)

I had completely redone all the brakes on the GMC 370, everything new, lines, mc, shoes, new hydrovac, wheel cylinders, except the 4 on the rear. At the time I could not find a part number for them. They didn't look too bad, so I honed them and put new washers.

Brakes were great. But after some time and puttering around the place in it, one of the rears began to leak. So I did find new ones at NAPA. Installed them and when the wife could go out to the farm with me. I had her to pump and I bled the rears. Hoping I would not have to bleed the whole system again.
Well, we got some pedal but not like before. We had to go, so I was going to wait till later to do it all.

Now, here is the kicker. Went today by myself and thought I would go back over the adjustment. Rear end was on stands, no tires. On a whim, I started the engine up and put it in high gear and spun the drums and pumped the pedal a few times. Thinking I would seat the shoes before adjusting again (2 per wheel)
When I went to adjust, the brakes were locked, so I started backing off on the adjusters, still locked.
So I opened the bleeders real quick and back. Walla!!
It bled itself. I proceeded to make the correct shoe adjustment. Put the wheels back on and I have great pedal and perfect brakes. How did it bleed itself?

If the rear end was on jack stands and was higher than the master cylinder, the air went to the highest point. The wheel cylinder.
Just a thought.

I don't know. It was jacked up just enough to get the wheels off. Not much higher than if on the ground.
It's a mystery.
I was thinking maybe after killing the engine, the Hydrovac was still holding pressure, since I had air in the system??

Sounds more like the brakes were adjusted too tight, especially since they locked up on you. That would cause the fluid to get hot, and be under some pressure when you opened the bleeders.

Maybe the reverse of what Tommy is saying. Maybe the air made its way to the higher master cylinder?

After putting on new parts, did you bleed the entire brake system, following the procedure for a system with a hydrovac unit?

Tim, when I did them the first time, with the rebuilt rear w/c, I did go by the manual procedure, starting with the hydrovac and ending with the hydrovac.
But this time, after installing the new w/c's on the rear. I actually just bled them. This is when the weird stuff started.
Now, I am back to the point of the complete bleed by the manual.
(Just waiting on a "pumper" now.)
Bill, I do have them adjusted with a slight drag. About as tight as I dare go.
Tommy, You have some good points there. It's very possible. Anyway, I think everything is back to normal. I do have brakes and the wheels turn freely. So, I think I am back to a complete bleed, to regain my full pedal.
Thanks to all.