The easyest way I have found to check the Hydrovac is to push on the brake pedal and then start the engine. If it is working you will feel the pedal fade down a little bit as the vacuum increases. Don't forget to add a small amount of 10w oil (3 in 1 oil should work also) to the Hydrovac canister from time to time. This keeps the leather seal soft. See the repair manual for instructions.
The breather cap found inside the cab is connected to the atmospheric side of the hydrovac canister. They were put inside the cab to help keep dirt from getting sucked in. My BIG trucks have this line connected directly to the clean air side of the engine Oil Bath Air Filter.
Mike
