Piston compressor runs but won't build pressure (stalls at low psi or tank stays at 0)

CheckClose the tank service valve so nothing downstream draws air. If pressure now builds normally, the problem is downstream leaks or demand. If not, spray soapy water on all tank fittings, drain valve, safety valve, gauge, pressure-switch unloader, and check-valve joints.

FixRepair leaks; replace a drain valve that's left cracked open (very common).

CheckAir hisses from the unloader the whole time the motor runs.

FixReplace the unloader valve or pressure switch.

CheckRemove the filter element and run briefly. If performance noticeably improves, the filter was starving the pump.

FixReplace the element. In dusty shops, check monthly; consider remote intake to clean air.

CheckHold a strip of paper (or your hand) near the intake while running: air being pushed back out the intake = leaking intake valve. A discharge line much hotter than normal, or air flowing back into the intake when stopped = leaking discharge valve. The head runs hotter than normal; the pump makes a 'breathing' sound at the filter.

FixLOTO and drain. Remove the cylinder head; inspect the valve plate. Replace broken reeds, the valve plate, and both gaskets (head & valve plate). Torque head bolts in a cross pattern to spec and re-torque after warm-up.

CheckSoap-test around the head while running — bubbles at the head joint. Squealing or hissing from the head. Air blowing between high- and low-pressure sides on two-stage pumps (intercooler relief pops).

FixReplace gaskets; check head and plate for warping with a straightedge; replace if warped.

CheckRemove the oil-fill cap/breather while running: heavy air blowing out the crankcase is blow-by past rings. Oil consumption is high and oil spits from the breather. Pump-up test comes in below ~60% of rated.

FixRebuild with rings (and hone/replace cylinders if scored) or replace the pump. For an old low-cost pump, replacement is often cheaper.

CheckSquealing on start, glazed belt, motor pulley turning but pump slowing under load. Check pump RPM against nameplate.

FixTension or replace belts; verify pulley sizes match the pump's rated RPM.

CheckMeasure voltage and amps under load. Low voltage = low RPM = low output and overheating.

FixCorrect supply; remove extension cords; use correct wire gauge.