I Have a 5.5.hp oilfree air compressor that builds air up to 45 lbs an stops building pressure and keeps run.will not shut off

Has the air filter been cleaned?
Does it stop running when satisfied or keeps going never satisfied with air pressure amount?
It could be worn and can't pump pressure it did as when new.

If the compressor is older or had a lot of use the rings might be worn. Try a compression test on the cylinder to see if there is bypass in the rings.Try this onlt if it keeps running and doesn't build pressure. If it builds up and stops at 25 lbs. the problem is different it would be in the pressure switch. You can then try to increase the the shut off pressure in the switch by turning up the adjustment screw. if this doesn't work I would then change the presure switch for a new one.

The check valve is not holding the pressure back off the head,therefore allowing the pressure to build up fully.

Small leak will prevent the unit from building proper pressure. Check all connections with soapy water and repair as needed (including under cover). If no leaks are found check cylinder for scratches. Push piston to top of stroke and check walls for fine scratches. If scratched, pressure is leaking past vinyl piston ring. If no scratches or leaks, most likely reed valves, head gasket or combination of both. Easy test, with compressor running and attained 90 lbs, place finger over air intake and check for good suction (carefull could be hot) . If good suction, then air is leaking somewhere, of no suction suspect reed valves/worn cylinder, if air seems to be blowing out of intake, suspect reeds /gaskets. Good holidays

sounds like valves in head have failed and need to be replaced, that is where I would start looking first.

Check to see if air is leaking past 'pressure relief' valve. It's probably leaking and that's where all the air is going.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/craig_3fa289bf857b1a3c

sounds like the regulator is fualty.

I am assuming that your compressor is oilfree type. Lets go over some basics to determine where fault is located. You will need to do some simple tests. If there are leaks the compressor may not reach set pressure. I use soapy water on all connections and on the head/headvalve (remove plastic covers to get to all parts that may leak) while compressor is running. Make repairs as necessary. If still low pressure, while compressor is running and has reached max obtainable pressure (in your case 15 lbs), carefully place finger over intake port hole (remove filter). If the compressor is pumping normally but leaking there will be ample suction at intake and cause the compressor to speed-up when intake is obstructed with finger. If you feel plenty of suction and no visible (soapy water) leaks, then the problem is probably the cylinder /ring is not correctly seating /scratched and air is leaking past cylinder. Make sure that the cylinder /piston ring is properly aligned and that the lip on the vinyl ring is up toward the head. On oilfree compressors you can examine the cylinder by pushing the piston all the way up, then look for scratches neer top of cylinder. If scratched install cylinder kit. If you do not feel suction but rather air blowing out of the intake, then the problem is the headvalve plate/reed valves are leaking. The reeds must seal perfectly on their seats and not be bent or have obstruction. Replace as necessary. Only one other thing to check and that is the intank check valve. Easy to check. Run compressor till you have about 15 + lbs in the tank then unplug compressor. Carefully loosen the inlet hose at checkvalve and listen for air leaking out of tank. If leaking replace checkvalve. Use caution when removing head to try and save the gaskets. If you see any model numbers on your compressor beginning with WL, you probably have a unit built by Campbell Hausfeld and can find parts at www.chpower.com
Good luck on your repair and email if you have other question.

compressor will not build pressure

Check the reset button on the motor to see if it tripped. The motor may be overheating. Some motors have an internal breaker and they have to cool off before they restart. Also check the pressure switch. Move the switch back and forth ( on and off ) and listen for a click. If it doesn't click with low to no air pressure then the switch is bad.