I have a craftsman air compressor model # 921.16472S. When it got to cut off pressure the bleed valve started blowing. I changed the pressure switch and bleed valve then lowered the cut off pressure. When it got back to cut off pressure the bleed valve blew again. What could be wrong with it

Your tank check valve is stuck/broke. It's on the tank end of the line from the pump to the tank. It is supposed to close when the pump stops to keep air in the tank and let the system bleed pressure out of the pump so the motor can start when there is pressure in the tank. The part you need is CAC-437-2 and is available through searspartsdirect.com.

If the unit does not unload properly the motor will have difficulty starting. Make sure that the intank check valve is not leaking air back out of tank (with pressure in tank, remove hose from compressor at tank checkvalve and spray soapy water). Replace as needed if leaking or try to repair. If the valve is not leaking then issue my be unloader valve at the pressure switch. Follow small airline from checkvalve to pressure switch. Small valve stem at the pressure switch should release air in line when pressure switch cuts off. Listen for small amount of air to be released when unit shuts off, if air continues to leak out, then check valve is leaking. Replace with generic parts making sure of fitting size. Good luck

The problem is lilkely the check valve between the compressor and the tank -- it is alowing pressure to build from tank to compressor preventing the compressor from turning over. Clean, maybe replace, check valve...

Charlie

Sounds like the unloader valve is not working properly. The unloader typically is at the end of a tube going to the pressure switch. The unloader is tripped when the pressure switch cuts off. The purpose of the unloader is to bleed the pressure from the cylinder when the compressor cuts off so the next time the compressor cuts on it will be starting in an unloaded condition.
when the compressor cuts off you should hear a discharge of air for about 3 to 5 seconds if the switch is working properly. The unloader can be adjusted if necessary by loosening the jam nuts and adjusting the unloader towards the lever on the pressure switch.

That is likely the compression release. A pintle valve in it should close while the pressure switch runs the compressor. After the switch cuts off, the valve is released to bleed the pressure off the cylinder head of the compressor to allow it to restart without having a load on it.

When the pressure switch cuts off you should hear a woosh as it bleeds but it should NOT bleed while it is running. Take it apart and clean the seat. Be careful of springs that go sproing.

there is a valve under the regulator, at switch off pressure the valve must blow of the air between cilinder and checkvalve after stop running.. when checkvalve is not closed it keeps blowing.

you have a valve problem in head /or leaking head gasket /or a bad leak in system
take off breather and see if it is pulling air it sould not be pushing back
one other thing would be the unloader at pressure switch or tank check valve

The air is probably coming through the unloader on the pressure switch. This is designed to vent the pressure between the compressor pump and the tank so that the compressor starts easier. This venting should only take a few seconds (5 to 10). If it bleeds air continuously, until the compressor restarts, then the tank check valve is bad. This is usually a fitting that extends into the tank. The small tube to the unloader usually goes directly from the pressure control switch to the tank check. Depressurize the tank, remove the check valve, and try to clean it. If that doesn't work, replace it.

It sounds like the unloader valve it not working correctly. The unloader valve bleeds off the pressure between the pump head and the tank check valve. Without it every time the compressor starts the piston would be working against the max tank pressure. This would cause what your describing. However when the unloader valve is not working the pump will shut off with in a couple seconds. If your pump is working longer then that I would guess that the unloader valve may be bleeding off some but not all the pressure.

On most smaller craftsman compressors the unloader valve is comprised of a piece of tubing running from the cylinder head or the check valve to the switch. At the switch you will see a plastic tip with a pin running to a small trip lever. Make sure that it's moving freely and moving through it's full range of motion. It's impossible to describe how this looks and feels to someone who has never messed with one of these.
After you have checked it and made sure it's working, if you still has trouble after this please let me know.

Most compressors have a check valve installed in the tank that allows air from the compressor to enter the tank and not allow back pressure to the compressor. Also there is an unloader valve at the pressure switch that activates when the pressure switch cuts out releasing the pressure in the line. These two devices allow the compressor to start in an unloaded condition so that the motor will not strain. When you place you finger on the intake, the compressor is no longer pumping air thus there is not load on the motor. Your problem may be the intank check valve or the capacitor may be weak or bad. Simple check for both. Many volt meters have capacitor check and amp draw. The capacitor should not be more than 10 percent or rating. enjoy