Remove the air from the compressor using the drain valve.
Remove the discharge hose. The one that goes from the pump to the tank.
Remove the air filter (and check condition)
switch the compressor on an with your hand, partially cover the air intake where the air filter bolts on(be carfull not to fully cover it).
You should feel the pump drawing air in.
If it dosn't draw you have a broken/leaking valve or a blown cross-over gasket if it is of the type with a split valve plate.
You'd need to remove the head/valves/plate to confirm this.

If it did draw in air,
Stop the compressor & re-fit the discharge hose.
Now run for a few seconds and check the draw again.
If its suction reduces you also have a valve/plate/cross-over gasket problem.

If it draws fine.

stop the compressor and loosen the discharge hose at the tank end.
You may hear a short air leak at first, but you should not hear air contniously leaking.
If it continiously leaks air then the check valve in the tank is shot.
NOTE: NEVER remove this valve if the compressor has air in the tank/receiver.
This should locate your problem for you.
Regards