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Some pressure switches have a valve that reliefs the compressor head pressure when the motor stops and closes the head pressure relief valve when the motor runs.
There is a check valve that keeps air pressure from coming back to the compressor head and the relief valve.
I think the check valve is leaking tank pressure to the relief valve and compressor head.
The check valve is where the pressure switch mounts on to and it mounts threaded on to the compressor tank.
Try replacing the check valve
The over-pressure relief valve may be defective if the valve is opening before the desired tank pressure is reached. If not, then the shut-off pressure adjustment (if there is one) may be set wrong. You'll need to find out from the manufacturer exactly what static tank pressure the compressor is designed to provide and then determine if it is working correctly. If so, then the relief valve might be defective. DO NOT attempt to defeat the relief valve under any circumstances - it can result in compressor damage or a dangerous tank failure.
Have a look at the check valve (you will find it between the pressure switch and the tank) Make sure there is no pressure in the tank before you remove it!!! There should be a spring and and rubber or plastic valve in there. Clean it up, or if necessary, replace it. Otherwise, you may need to check the valves in the head, if the are not seating properly then you will get the air passing through. It is very irregular for the air to get passed the piston rings in the pump, but try those things and post on here to let me know how you get on
You didn't give the model of your compressor so, for an electric model: Check your gages while the tanks are filling. If the pressure gets to the rated high pressure level for your model and keeps going until the relief valve pops, the problem is with the pressure switch. It isn't turning the motor off when it's supposed to. If the relief valve pops before getting to the max pressure the tanks are supposed to be at then the spring in the valve is broken or weakened to where it pops too early. Replace the relief valve.
I can not find a Craftsman compressor with that model number anywhere so here are a couple things to look into. If the compressor fills to 70psi and keeps on running check the valve plate and piston/cylinder assemblies. Check that the valve reeds on the valve plate are not bent, broken or warped from heat. Then check that the wiper (oil less) or pistion rings (oil splash) are not damaged and that the cylinder wall is not scarred up. Any of these will let the compressor fill up to the point where the air pressure leaks past the valves or piston and won't fill any further. If the compressor fills to 70psi then shuts off the problem is in the pressure switch. Some have an adjustment screw or bolt to control when it turns on and shuts off. If you had a drop from 100 to 70 the springs may be weak or broke and you'll have to replace the switch.
If the spring in the relief valve is weak or broken it will let air out well before your tanks get up to pressure. You can test it by putting pressure on the end of the valve to hold it closed to see if the tank pressure gets high enough to switch the control valve open and idle the motor down. Watch the tank pressure so you don't exceed its safe level. They're relatively inexpensive so you can also just replace it to see if it fixes your problem.
first you have a reciprocating pump and the hissing you ear probably comes from the unloading valve on the pressure switch(it will look like a bike tire valve)if there is air coming out the valve when the compressor is off it probably means you have a bad check valve.This valve is use to unload the pressure inside the pump for an easy start .a motor always drive more amps on start up if you cant unload this pressure you will trip the overload(you probably have a reset button on your motor)check the specs on your pressure relief valve ,its a small pump maybe its just adjusted to high.in this case your pressure relief valve will open .If the pressure is fine and there is air coming out the relief valve then you have to replace it!to adjust your pressure turn the screw on your pressure switch clockwise to increase and counterclockwise to decrease .usually you have a 30 psi between cut-in and cut-off called the pressure differential.It can be adjusted on some pressure switch.be sure you put some teflon tape on the treads if you replace something to avoid air leaks.hopes this help!
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...
The most likely problem is a burned out "run capacitor" on the motor. If you continue to try to run your compressor in this condition it will burn out the "starting capacitor" and/or motor as well.
It's also possible that you have a bad start up pressure relief valve, this valve will be between the compressor and the tank. If this valve is working properly you should hear a hiss of air as the compressor motor shuts off.
A reasonable motor repair shop should be able to replace the run capacitor for $20-$50 plus inflation, depending on your area.
If you need to test the start up pressure relief valve, you need to take it loose at the compressor or tank. Just loosening the line may not work depending on the style used. Note: sometimes just taking this loose to test it causes you to have to replace the valve.
the problem is in the check valve where air enters the tank.when leaking it lets air pressure up to the air piston causing the motor to stall upon restart.replace check valve.average cost of part 15 to 30 dollars.simple fix