Your air compressor probably has something broken inside the cylinder. Disconnect the power to the air compressor and remove the plastic cover around the motor and compressor.
Before you take the compressor apart, listen and feel for air leaks. Check to see if air is coming back out of the intake filter. If air is coming back out of the intake filter, something is wrong inside the cylinder.
You can see a parts diagram at [searspartsdirect.com
]. The parts diagram will help you locate the parts that build air pressure. These are the parts you need to inspect.
Turn the motor shaft by hand and be sure the piston goes up and down.
If the piston is ok remove the 4 screws holding the head on top of the cylinder. Remove the head.
Directly under the head is the pressure plate. The pressure plate has reeds on it that direct the air into the discharge line and keep the air from going back through the intake side of the compressor. Check the reeds for warping or breaking. If the reeds are damaged, you must replace the pressure plate. Make note of how the pressure plate sets and the o-rings that seal around it.
Remove the pressure plate and inspect the cylinder walls for scratches or wear, if the cylinder is damaged you need to replace the cylinder, piston, and rings. Make note of the piston and the rings on it. If a ring is worn or broken the air goes back through the intake filter and no pressure is built. If you have to replace the rings, replace the piston and sleeve also.
To replace the rings and piston, you have to remove the nuts that hold the piston to the eccentric.