[Bob B.]

A Canadian 940 is the equivalent of a U.S. GMC C-4500 or Chevy C/40.

You have either a 307 or a 350, with the 350 being more likely. Look for a 2 or 3 letter code stamped in the block in front of the right cylinder head, and look the code up on line. Yes, that truck probably has vacuum boosted brakes. You cab see the vacuum hose going from the intake manifold to the pipe that goes to the booster next to the master cylinder. The vacuum booster is probably behind the left running board under the cab, and there should be a breather for it under or behind the driver's seat. It could be your booster has a bad diaphragm or control valve in it, which would explain the hissing noise and poor braking performance. These systems actually work very well if the booster is good and the brakes are adjusted properly.

Nice little truck!