Air compressor 921.166400 craftsman. Wont build pressure over 45 lbs. Hear a small air leak from small tube under pressure gauge. Any thoughts

Try carefully lightning the tubing nut to resale.

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

It sounds like a bad gasket. Either the one between the valve plates or the one between the head and top valve plate. The top gasket is the most common one that fails. If the middle of the gasket is broken, where it separates the intake from the exhaust, you will get symptoms just as you described. Will only build up a small amount of pressure. Here is a link to look at the parts diagram and to order parts from.

http://www.searspartsdirect.com/partsdirect/part-model/Craftsman-Parts/Air-compressor-Parts/Model-921153101/0247/0703000/50037535/00001?blt=06&prst=&shdMod=

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

when the compressor is running do you hear air blowing off? If so, check the manual blow off valve to see if it stuk open. The regulator controls the pressure check it for leaks

The compressor will not auto shut off until it reaches the preset pressure which is usually around 100-130 psi - your problem is leakage of air from the oil filler cap suggesting that you have a failed internal valve.

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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sounds like the regulator is fualty.

The pressure switch will shut off when the set pressure is obtained. Usually about 100 to 120 lbs. If the compressor stops at lower pressure than mentioned you may try to adjust the pressure by looking under the switch cover for adjustment screw/nut.(no more than a few turns) Increase pressure by increasing spring tension. Quick cycling is usually caused by air leaking from tubing, drain valve or check valve. After compressor shuts off unplug it from the outlet and spray soapy water on all fittings and drain valve. Repair leaks and test for quick cycling, however the compressor should reach 100-120 before shutting off. Some small compressors have a thermal reset but that usually takes a few minutes to reset. Pressure switch is defective if it shuts off before setting mentioned. Enjoy

If this compressor is dry type with no oil needed, then it may have a worn cylinder and/or piston ring. With flashlight in hand, look under cylinder with piston in upper most position. If you see scratches most likely it needs cylinder / piston kit. Not to worry, not too expensive. If you compressor has a crankcase with oil, then you may have problems with valves in the head. Cracked or dirt under headvalves will prevent pressure from building. Finally there may be one or several leaks in the tubing, check leaks with soapy water. enjoy