I have a craftsman 20gal. 1.5 horsepower compressor. I turn it on and it runs forever and there is no pressure in the tank. what sould to First.

in some cases there is a small filter inside the tank, try blowing the line out backwards to the tank with an air compressor'' light pressure''

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

if its belt driven ,unplug it(safety first)remove the belt and try to turn the pump by hand.if the pump turn plug it back without the belt. and try it .if the motor doesnt run.check the contacts on your pressure switch.if they are ok and try again.if not you need a new motor.if the motor ,the pressure switch contacts,and the pump are fine.put the belt back back on bleed your tank to zero with an air gun and try to run it.if it work well till it cuts of check if your bleeder release air.you might have pressure stuck in your pump.the compressor needs to release pressure for a soft start.hope this help!

Without the model number I am not for sure what pump you have but there are 2 things that can cause this.

#1 Would be the check valve in the tank may be weak or bad. This allows air to go into the tank and not escape. If this is bad typically you will have air coming back out of your pressure switch after the air compressor is turned off.

#2 You may have a blown head gasket between you first and second stage in the top of the compressor pump. To solve this problem you would need to replace the head gaskets in your air compressor.

Thanks

Does it run? Does it stop running? Does the pressure gauge read anything at all? Is there a regulator on it? Can you take the reg off? A bunch of things to check, get it running and let me know what you find, and I will try to tell you what's next.

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

Reciever tank holed

Compressor Piston rings worn

Will run forever because it never hits the limit on the pressure switch

Check the Reciver tank connections using a strong mix of washing up liquid and water

Apply this around the connectors and see if they blow bubbles

Check the underside of the tank in the same way , that is where the water sits and that is where the rust starts

If you cannot fins a leak you are back to the piston rings and head valves

TRY ADJISTING THE TANK PRESSURE KNOB WHILE RUNNING ,TURN TO THE RIGHT 1/4 TURN UNTILL YOU SEE NEEDLE MOVE .MOST TIMES FACTORY SETS AT 0,SO YOU NEED TO ADJUST IT .MY ADVICE IS NO MORE THAN 90-100 PSI TILL YOU KNOW WHAT YOUR RUNNING MORE OFTEN

Double check that the drain valve is fully closed on the bottom of the tank. Once it reached 100 PSI, did it stay pressurized or did the pressure drop? Did you run the initial break-in procedure described in the manual? Do you hear any air leaks?