The best way to test for air motor leakage is to totally seal off the
large supply nipple and turn the motor crankshaft backwards.  If the
motor is not leaking, it will be very difficult to turn.  If the motor
leaks, it can be turned backwards with ease.

Testing via plugging the hose to the air motor and pumping will not
accurately diagnose a leaking air motor because the pumping bellows and
lower mechanism may also have leaks.

Bruce Clark