Brakes lock after Hydrovac repair! CCKW353

[cckw-353.de]

Hi guys

Over the last 6 month I was loosing brake fluid into the nirvana, I thought! No spot under the truck (apart from engine oil) but every 6 weeks or so the master cylinder was empty. The only way it could disappaer was the main shaft seal ov the hydrovac. So today we took it out and put an overhaul kit in. There was actually about 1/2 liter of BF in the vacuum part of the Hydrovac. I have now installed all new seals. After putting it back in and a quick bleeding I started the engine, rolled forward and tipped the brakes...a sudden stop, all 10 blocked and the brakes won't let loose again
What's wrong here. pleaese. Does the Hydrovac need adjusting? I have all 5 GMC manuals from the Austrian Army, all in German and everything in there down to overhauling the engine and all the gear boxes but the hydrovac is only been spoken about as a whole unit no hint about overhaul or adjustment. Please help me...!!!

thanks
Uwe

[Karoshi]

Hello Uwe, Ihr Englisch ist sehr gut.

You have a very fine looking truck. The fault you have on your brakes could be for one of two reasons, but first what model Hydrovac do you have ?, a series 1, series 2 or series 3 ? How many rubber seals did you replace in the rebuild, was it three? Two small 1/2" ones and a single 3/4" larger one?

If ALL of your wheels lock up, your problem is most likely in either the shuttle valve of the Hydrovac or the master cylinder.

There is a VERY SMALL hole in the master cylinder that is prone to blocking. If it is blocked it wont allow the master cylinder piston to return and relieve the pressure, so your brakes stay locked on.

Please give us more information.

[cckw-353.de]

Hello again

I have no clue what Hydrovac version I have. Please check the pictures below...I don't think it's the main brake cylinder. The brakes worked until yesterday even with slowly loosing fluid. I heard from friends in the UK that had simular problems with Hydros in their M-8s! After re-building them they locked up. I was told that a certain adjustment is needed...just don't know what/how?








thanks
Uwe

[Karoshi]

Uwe, there is no adjustment on a hydrovac.

[cckw-353.de]

Hi again

The following parts were replaced:
-Seal at the end of the Hydraulik part (Simmer Ring?)
-Seal behind this, 1inch diameter, the same as on piston
-Piston seal 1 inch
-small seal on small piston that is connected to the membrane about 1.2cm
diameter
-Membrane unit
-double seal dishes that are connected with flexible shaft, 3cm diameter and sit under the lid at the side of the unit closed by 6 screws...

What make me thinking is that the distance between the two dishes on the replacement was a bit closer than on the old one! Is this the shuttle valve? I do not have the Hydrovac manual so I do not know what the parts are called.

thanks
Uwe

[dr deuce]

If you had BF in the hydrovac, clean your check valve on the engine. It is most likely all gummed up and will admit gas fumes to the new unit and damage it.

[Hardbird]

This same thing happened to me on my half-track.

Make sure that the large leather diaphrams and their backer plates are are mounted in the correct position. They need to be in a certain position as to allow the rod that is on the end of the diaphram shaft contact the relief valve.

If these plate/diaphrams are too close one way (I cannot remember which way) the rod will not release the valve. They need to be set so that the plunger rod goes as deep as it can into the bore where the release valve is at the front of the hydro-vac. This is the release valve in the hydro-vac, not the master cylinder. If the position of these are off only by 1/8" of an inch the release valve will stay closed in the hydro-vac and thus keep the brakes locked for a period of time or until the pressure is released at the front bleeder on the hydro-vac.

The rod that I am talking about slides into the bore in the picture you have labled as ResizeofP1019557.

Ron

[cckw-353.de]

Hello all

I have fixed the problem. It was the poppet valve. The overhaul kit I had was obviously for a slightly different Hydrovac model. The seals for the cylinders and the seal at the end of the hydraulik part were the same but the two disks of the poppet valve were closer together when installed. This cause the timing in the control unit to be off. I swapped back to the original one and...voila...problem solved. The Hydrovac is now working just fine and with all new seals in the hydraulik part I expect the brake fluid to stay where it belongs. I did not touch the large pistons in the vacuum part, just regreased the leather with vasiline and cleaned the cylinder halfs.

thanks a lot

Uwe

[MotorpoolMac]

Uwe – How did you get the poppet valve on and off? I just rebuilt my hydrovac and twisted off the flex shaft thinking it would unscrew.

Thanks!
Daniel