I have a Craftsman 1 hp 5 gal air compressor. Worked fine last summer, but now will not get above 20 lbs pressure no matter how long it runs.

check air tank drain valve located on bottom of tank, if it is not leaking try adjusting air pres regulator (large black knob close to air hose connection.) the knob when turned in will increase the tank air pressure, if knob wont turn check that lock ring on bottom of knob to make sure it is disengaged. also check for air leaks in all lines and connections

If it makes hesitation sounds just before shutting off, you need to oil the compressor as per the manual. This is the most common problem.
If it is kicking on and off normally, you need to adjust the pressure relay. The instructions are under the cover.

Oil free pumps do tend to blow the piston ring (not too expensive) available at searsparts.com. Remove covers to pump and manually turn crank until piston is at top and look at the condition of the cylinder neer top, If it looks scratched, order cylinder kit with piston ring. If it looks good and shiney, could be the valve plate in the head. There are reed valves that tend to break. Run pump however with covers removed and check with soapy water for air leaks and repair as needed (air leaks under the cover will prevent unit from filling). Good luck

Maybe your unloader valve is stuck or damaged. This is found in the switch assembly and if it is faulty you should be able to hear a constant hissing noise from the switch .

If your motor is running and the pressure is not building it is either the valve plate or the compression ring. Check both for damage.

Check to see if air is leaking past 'pressure relief' valve. It's probably leaking and that's where all the air is going.

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The compressor valve may have a faulty gasket or seal, and maybe leaking air internally. Or the compressor might be going downhill. You can try to replace the valve seal and see if that makes a difference, otherwise you need to replace the compressor.

sounds like the regulator is fualty.

you have to adjust the regulator usually turning clockwise increase the pressure.

I am assuming that there is no air in the machine now. Otherwise it will not turn on if there is enough pressure in the tank.

If it will not start at all, the on/off switch on the pressure switch may not be turned on or it is defective.

You can also check the reset switch on the motor (red button).

These are very simple setups and it shouldn't take too long to troubleshoot.