The check valve should be just a brass plug that sits inside of a small housing with 4 machine screws. Take it apart and clean all the gunk out of it. The brass piece should be round with a little 'dork' in the middle pointing up. So long as the seat on it is smooth and the seat in the bottom portion of the housing is smooth it should be fine.
The way this works is the vaccum sucks that little piece up in the air letting vaccum go to the hydrovac. When the vaccum stops, gravity lets it fall down into place giving you a little stored vaccuum for one more easy push on the brakes.
A good way to test your unit is to remove the plug on the back of the Hydrovac and hook up a vaccum gauge. Start the truck and let it run for at least a few seconds. Turn the truck off and it should hold around 17 (+ or - a few) pounds of pressure. If it doesnt hold any or very little you have a vaccum leak somewhere in the system.
Hope that helps!
Good luck, Jeff
