The Stovebolt Site Forums The Parking Lot Big Bolts Help ! Big Bolt Vacuum problem

I've looked all through the service manual and can't find anything that explains what's in the vacuum system, so I was wondering if you guys know what's in the system.
I know theres a check valve which I'm guessing is mounted on the firewall. The check valve was mentioned in the Hydrovac function.
The 2spd axle switch/valve is mounted on the frame in a handy easy access point above the exhaust pipe right near the clutch rod.
Looks like it would be easy to get to once the motor and body were removed.
My main question is are these parts repairable and whats inside them.
The Vac leak that I've got I cured for the moment by plugging off the main Vac source at the manifold.
So,My fellow Bolters....Has anyone been there, Done that ?
Dan Oh, It's a 52, 6100, 2 ton Chevy

Windshield wipers

Check the hoses and fittings for leaks, my soft lines were really worn down and the main line needed to be wraped to get it to stop leaking. Best method is a small bottle of soapy water, shake to snot out of it and put the resulting foam on the suspect areas. If there's a leak it will suck the bubbles into the leak.

glasman,

It sounds like there are only two circuits on the downstream side of the intake manifold, the hydra-vac and the two speed rear end.

I have never seen the check valve for the hydra-vac fail before, but the diaphragm in the hydra-vac might be bad.

If you start the rig with all hoses connected, then shut it off, you should have a 'powered' pump or two left in the brake system.

If you do, this means the system is vacuum tight, everything is working and the problem probably lies with the vacuum two speed shifter.

The conventional method to find a vacuum leak is with some type of spray can containing something more volatile than gas...like carburetor cleaner or brake-kleen. When you squirt this on a fitting or connection that is leaking, the material is drawn into the engine and you will notice a rise in RPM.

Using this method you don't have to have a clear visual path to see bubbles disappear.

Stuart

The hydrovac check valve should be located close to the the intake manifold. Eventually over time it will get plugged up with oily residue and will need a good cleaning. You should be able to follow the intakes tube and find the check valve relatively close by. Unbolt it from the line, pull it apart and clean it up. Its a brass plug that lifts up from the vaccum pull. Once the engine is turned off, gravity works and it falls downward sealing in a little bit of vaccum pressure for one more stop of the brakes.

If its pitted or gouged youll need another one. Otherwise just clean and polish it back up and stick it back in. The best way to check whether or not the vaccum leak is coming from the hydrovac itself is to take the fill plug off of the back of the power cylinder and hook a vaccum gauge to it. Start the engine for a few moments. Turn it off and check the readings. If everything is in proper order it should be holding 18-22 pounds of pressure. It will eventually fall.

As far as the vaccum shift for the rear end, youll want to follow all the hose connections from front to back to find the source of the leak if thats the problem. I like atomarcs idea of using carb cleaner and hearing the rpms rise.

Let us know what you find.

Jeff

Does anyone have an idea of whats inside the 2 spd axle switch thats down on the frame.
Stuart, thanks for the hint on the check valve.
I don't really have a problem with how to locate a vac leak, I was wondering what to expect in these two main parts of the vac supply system.
Dan

Heres whats involved with the hydrovac function and how it works.

I havent dealt with a two speed rear yet, so Im no help there.

Jeff

you can plug the vacuum line going to the two speed switch and see if that stops it.. as far as switch itself if i remember right there are just o rings in it,, also the two speed (switch motor) on the rear end itself has a diaphram in it that could have a hole in it, you can take the lines off at the rear end, and check them with the switch in each position to see if it is drawing vacuum,, also quick way to check that diagphram is to just suck on one of the ports going into it.. to see if it will hold vacuum.

One less-messy way to find a vacuum leak is to use a propane or acetyline torch. Just turn the fuel on, but don't light the torch. Pass the tip around the suspected leak areas, and when the fuel gets sucked into the leak, the engine speed will change. No messy liquids to clean up, and propane doesn't dissolve paint like carb cleaner.
Jerry