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

My 1953 Chevrolet
1947.1 Gallery
1972 C-10 1/2 Ton & 1972 C-30 1 Ton