If you're checking the Hydrovac for brake fluid by removing the small plug on the back of the piston can you're looking on the wrong side of the piston. That side is the fresh air side and the vacuum side is between the piston and the hydraulic cylinder so the fluid may be trapped and the piston seal is keeping it out of the fresh air side. If the vacuum side has fluid in it the engine will smoke as it sucks it up the vacuum hose. Remove the hose at the engine and see if it's wet on the inside.
Mike B
