Air compressor won't build pressure

Symptoms

• Gauge needle stays low or climbs very slowly
• Motor runs constantly and never shuts off
• Hissing sound from fittings or the tank
• Tools run weak even with the compressor on

Likely causes

• CommonAir leak at a fitting, hose, or quick-connectLoose or worn fittings let air escape as fast as it's made.
• CommonWorn check valveThis valve should hold tank pressure when the motor stops; a bad one lets air hiss backward through the pump head.
• PossibleWorn piston rings or reed valves in the pumpCommon on older or heavily used compressors and reduces how much air the pump can push per stroke.
• PossibleDrain valve not fully closedThe petcock at the tank bottom is an easy leak point to overlook.
• Less commonCracked or corroded tankRare but serious; inspect for rust bubbles or seams if the tank is old.

What to do

1. Unplug the compressor and release all tank pressure through the drain valve before inspecting anything.
2. Make sure the tank drain (petcock) is fully closed, since a cracked-open drain is a common hidden leak.
3. With the compressor back on and building pressure, mix dish soap and water and brush it on every fitting, hose connection, and the tank welds; bubbles show the leak.
4. Tighten any leaking fitting with a wrench, or replace a leaking hose or quick-connect.
5. Turn the compressor off once pressurized and listen at the intake filter and pump head for hissing; a hiss there often points to a bad check valve.
6. If the check valve is suspect, replace it (it threads in near the tank, usually inexpensive) and retest.
7. If no leaks are found and pressure still won't build, the pump's piston or valve reeds may be worn and need professional service or pump replacement.

Tools and parts

• Adjustable wrench
• Spray bottle with soapy water
• Rag

When to call a pro

If the pump itself is worn (rings or valve reeds), a small appliance or tool repair shop can service or replace the pump; on older units, a new compressor is often cheaper.

Safety