My Maxus ex8000 4 gallen conpresure will only pump up to75 psi, and wont shut off, and there are no air leaks.

here is a link
copy and paste

https://www.about-air-compressors.com/bostitch-wont-build-pressure/

replace the non-return valve in the pipe at the tank connection
air out of intake is poor poppet/read valve seating and pressure in the sump is the tank air leaking back and down past the rings
150 psi is excessive and not good for the compressor so adjust it back to 100 -110 psi

Check your regulator to make sure its set to maximum. When was the last time you drained the tank, it may be full of water.

Your tank check valve is not closing properly. It's a valve on the tank where the air line from the pump head attaches, It allows air in but not back out of the tank through the pump. When the pressure swtich turns the motor off, valve on the pressure switch lets the air out of the head so the motor can restart without any pressure in the pump head when the pressure in the tank drops. On rare occasion, you can clean the check valve out and it'll work again, more often than not you have to replace it.

Without knowing what make compressor you have;
- if it leaks air while the pump is running, replace the unloader valve
- if it leaks air after the pump shuts off, replace the tank check valve

Repair all leaks to get true test of condition of cylinder / piston ring and check/reed valves. It is reccommended to know amount of time to fill tank when new and compare this time to present. Filling time is indication of wear. Another method to discover wear is to remove cover to pump and examine condition of cylinder wall close to top of cylinder (on oil-less models). If you see scratches on cylinder, air is leaking past piston seal. On pumps that use oil, very hot discharge air hose or pump head is indication of leaking valves or head gasket. Sears usually has good prices on replacement parts. Good holidays

strip the cyclinder off and inspect the valves if closing and opening also check to make sure you dont have a blown head gasket also on the tank from the pipe from the pump unit is a gold valve you need to open the end off this valve with the tank drained and turn the ruber around with the spring inside and replace this if still doesnt work get back to me bud

If the pressure does not get pass 30 psi, then check for leaks along the pipelines and on the tank itself. An air leak on the top portion of the tank can cause the pressure not to trigger the pressure switch. Use soap bubbles to detect any air leak. Although a stuck pressure is not more likely the problem, check it also.

if it has a bowl on the bottom of the carb you float might be stuck, and check if the air cleaner is dirty

The locking hub control system is not engaging properly. On the 99 Tracker this is a compressed air system that consists of an air pump (compressor), electrical wiring to power and control the air pump, and air hoses that lead to an actuator inside the front differential housing. If there is a leak in the system the PCM will run the air pump for 10 seconds and then shut off, and your propeller shaft can turn all day long, but the front axles will remain free floating. Turn on your key, not your engine, and engage four wheel high. Go listen near the passenger front bumper for the air compressor. If you hear it, then the compressor control and the compressor itself are likely fine. If it shuts off after 10 secs or so, and your 4WD light isn’t on, the PCM timed out the compressor out and shut the compressor off to protect it. You will have to find the leak and fix it. If you are lucky, it’s the air line leading to the front differential; and not the air line inside the differential housing, or the air bladder in the locking actuator, which also resides inside the font diff. If the compressor shuts off and the 4WD light is on (this means the PCM did not shut the air compressor off due to timeout, so the pressure switch read 5-7 psi and the pump shut itself off), you likely don’t have a leak, and something mechanical has the actuator hung up inside the front differential. Lastly if you don't hear the compressor run at all, you either have a faulty compressor, or some sort of wiring problem in that circuit.....it could be as simple as a blown fuse. Good luck.