There are only 2 points of leakage when air checking with the pump. The seals on the piston and the large diameter rings on the reaction shaft (back side of the pump) that seal the pump to the drum. Are there grooves inside of the drum where they ride? If the drum is worn, they're cheap. I just bought a new Dorman drum at my supplier Thursday for $32. That being said the front clutch is always lazier during air testing than the rear drum. And yes it's easier to air check with the pump rather than assembling the unit.
Doug