The Stovebolt Site Forums The Shop Area Driveline Brakes again?

I have seen this discussed before, but cannot find it again. Brakes staying on> It's the 57 GMC 2 1/2 ton.
I had them working perfect, everything new, including new hydrovac. Now all of a sudden the rears are not completely coming off. The pedal feels hard too. They are holding the truck back when driving, and I cannot turn either side by hand with engine off and jacked up off the floor.. I have backed the adjustments off about 6 clicks and even opened the bleeder at the master cylinder on the hydrovav. It squirted some fluid out, like the hydrovac was still applying pressure to the rears. Still cannot turn them by hand. The fronts turn freely. Has something happened to my new hydrovac? I have not even drove it out on the road but a couple of times.

Hy Old GMC Gold, the hydrovac is a "slave" to the master cylinder, make sure the master cylinder is returning to its full at rest position. Check your pedal/linkage adjustment to make sure there is a small amount of clearance between the linkage and the master cylinder piston when the foot pedal is all the way up. I trust you have a return spring on the pedal/linkage. Try turning the rear wheels after they sit for 24 hours, before applying the brakes, hope that helps.

Thanks 3b. I will check all that tomorrow. I do have a return spring, unless it has come off. Before this happened, my pedal would go down a couple inches, with great brakes. Now it is at the top and feels hard.
What does that mean has happened, if they will turn after 24 hours?
Thanks much.

Hy Old GMC Gold, even if the master cylinder doesn't return all the way to the at rest position the pressure created by not uncovering the compensating port in the master cylinder may bleed off after 24 hours allowing the rear wheels to be turned.

Hy 3B. You were right! I went back this morning and the wheels were turning freely. It amazes me at the knowledge you guys have!
After seeing they turned freely, without starting the engine, I pumped the pedal a couple of times and it felt normal. The wheels still turned freely. THEN, I started the engine and pumped the pedal a couple times. LOCKED up again. Even the brake lights were on. (it has the switch inline).
I should have learned by now, not to take short-cuts. I honed and rebuilt the master cylinder myself, about a year ago. I do have the return spring on, and the correct free travel in the linkage.
I guess now I should call Midwest and get a new MC.
Thanks for your help, I am just glad I don't have to pull the rear hubs again.

Hy Old GMC Gold, now hang on here just a second, if things were "normal" until you started the engine, then I would think your Hydrovac may be the cause of your problem. The atmosphere valve may be leaking or stuck open, this would cause the brakes to partially apply as soon as the engine developed vacuum. Do you have a manual or schematic which covers the Hydrovac?

Maybe the vent for hydrovac is plugged up?

Yes, I have the service manual that covers the whole truck.
Are you talking about the filter that is mounted on the firewall that furnishes clean air to the hydro?
I had disassembled it and cleaned it when I first started on the brakes. Maybe a dirt dauber nest in it now?
I will get the manual out. Is the atmosphere valve inside the hydro? I will have to look at the manual to see where the vent is.
Could it be in the vacuum check valve?

Hy Old GMC Gold, the atmosphere valve is supposed to be activated by hydraulic pressure created by the master cylinder, the opening of the valve allows air under atmospheric pressure to enter the assist side of the bulkhead/diaphragm. The air under atmospheric pressure assists in applying brakes, the more you apply the brake pedal the more atmospheric air is allowed into the assist side of the Hydrovac. If the atmosphere valve is off its seat for any reason other than the way it was designed to be problems with too much assist or brakes applying when the engine is started can occur. If the fresh air (atmospheric) line or filter were plugged little or no assist would be felt, hope that helps.

OK. I have studied the schematics and manual on the 9 1/2 inch Hydrovac. I have a basic understanding how it works.
I am definitely holding atmospheric pressure, with the engine running.
It looks like they call it the control valve, just behind the inlet tube for fresh air.
So have we determined it's inside the hydro and not my MC?
If so, I don't want to go into the hydro. It is new (not a rebuilt one). I think I should get with the place I bought it.
There is a lube plug on the front of it. Maybe they did not put any lube in it? I have not took the plug out yet to check.
Don, I cannot find a vent listed in the breakdown of the parts in the manual.

The one on my '53 is one the frame and not far from the hydrovac. Sorry I can't help with the location for your '57.

Don, I think we are talking about the same thing. It is actually the intake for the fresh air? Mine is mounted on the firewall under the hood. You can remove the top and there is a small paper filter in there.
According to 3B though, mine is not stopped up. I am actually getting too much fresh air, because the valve in the Hydro is not releasing it back into the engine.

Hy Guys, the Hydrovac functions by using vacuum created by the engine running and hydraulic pressure created by the master cylinder when you step on the brake pedal. The assist is created when a hydraulically actuated valve closes off vacuum to the assist side of the bulkhead/diaphragm while simultaneously allowing air under atmospheric pressure to enter the assist side of the bulkhead/diaphragm. In the brake application situation you should have a vacuum (negative pressure)on one side of the bulkhead/diaphragm and air under atmospheric pressure (positive pressure) on the other side. This positive pressure easily overcomes negative pressure and assists in applying hydraulic pressure to the brakes. We seem to have a situation with this particular brake system where as soon as engine vacuum is created the brakes at least partially apply, which would indicate a problem with the Hydrovac, I would call the shop where you purchased the Hydrovac and explain the situation to them and see if they can offer any suggestions. Your brake system did function correctly for a period of time after you finished overhauling it didn't it?

You have summed it up correct, 3B. Yes, I have run it around the pasture and on a paved road 2 or 3 times. I think I started doing the brakes about 2 years ago. They worked perfect until this weekend.
I don't have a tag and insurance yet, so I have not given it a real good road test, but I have run it around the place almost every weekend.
I bought it from Rich at Midwest Power Products in Morris, Il.
He is a real nice guy and knows everything about brakes from probably 1890 to the present.
Yes, I will call tomorrow. I just want to be sure what I am talking about. Guess it's not in my MC, because they work fine and release good, without the engine started.
Thanks very much to all for the help.

Well, it turns out it was the MC. Rich at Midwest gave me some steps to check. First, was to get them locked back up, then crack open the line to the MC. If that un-locked them...it meant the MC was bad. Sure enough, as soon as I cracked the line open, the wheels turned freely.
Sort of glad it was the MC. I did not ever trust the old one, I rebuilt myself. But it did work for a couple of years.

If you have not installed a new master cylinder maybe you should try adjusting the rod that goes from the brake pedal to the master cylinder. Make it shorter. Then try it again? Just an idea. Don

Don, that might be a solution. But I have already ordered a MC. Got one from Rockauto for $95. I actually wanted to put a new one on it when I first started the brake job, but I did not know the sources that I know now. All the parts stores around here just told me no chance of getting a new one.
I am sure the one on it, is the original one and I had doubts when I rebuilt it.
I do have the correct adjustment on the linkage, but thanks to all for the help. (who knows, may still not be the problem)

Update! Got the new MC on from Rockauto. That was the problem. I bench bled it, installed it, and had the cousin to pump, while I bled the Hydro and slave cylinder. Did not even have to re-bleed the wheels.
Works perfect. Not locking up anymore.

Hy Old GMC Gold, great news, thanks for keeping us in the loop.