I have a Husky air compressor model WL651004AJ. The problem is that it builds pressure initially up to 75 psi then shuts off. I have to turn off the power switch, wait about 5 minutes, turn the switch back on before I am able to restart it and then it only will build up about another 25 psi. I have to repeat this process several times until it reaches 150psi. Does anyone have any idea what may be wrong with it?

Your compressor will work in the vertical position however you will not be able to properly drain the water from the tank in this position.

I have a Husky WL651004AJ which has not been starting up for family members lately and noticed a 50 ft extension cord was being used because of the distance from the power outlet. I moved the air compressor to the power outlet and plugged in the power cord without any other extension cord and the air compressor works fine. The 50 ft cord was too light duty for the power required.
(12 ga) and min gage of 10 for the distance of 50 ft without disrupting the power breakers.

I would suggest checking the valves . Make sure they are good,no trash ,seating properly. other items to look at include, head gasket,installed correctly? air filter plugged or inlet blocked with something

All of your problems indicate the pressure switch is bad. I don't know which part number to recommend because Campbell Hausfeld (Husky) model numbers have six digits and there aren't any model numbers or part numbers I can find that start with WJ. You need to contact Campbell Hausfeld (1-800-543-6400) to see what part you have to order to fix your compressor.

You hit the nail on the head ... it is the regulator. What does it say on the side? What are they advertising the pressure and the SCFM? (Standard Cubic Feet (per) Minute)

The boss recently bought a Husky. It is rated at 6 SCFM @ 200 PSI (as I recall) ... Its regulator turns the machine off at 240 PSI then delivers the air through a regulator. In my case, I throttle it back to 150 PSI so I can get 80 PSI at the end of my 75 foot long hose. The machine comes on around 150 PSI (or of course any lower number) and takes for ever to build pressure back.

Should you change the regulator setting? Only if you want to diminish the performance of the compressor and possibly void the warranty. If yours doesn't have a regulator, I suggest you add one.

I hope you have found my comments helpful ...

Thanks for visiting today.

If you can hear air bleeding off through the air filter when you turn it off then the check valve has failed on the compressor discharge to tank line and cannot overcome the tank pressure. If there is no air coming out through the air filter than the intake and dischare valves have become weak or are broken/not seating. Post your findings, I wil see if i can help I worked on compressors for 5+ years

You can inspect the check valve by removing it and pulling on the small plastic pin sticking out of the middle of the tank side. It should spring back under fairly light tension. Also visually inspect and clean out with a dry q-tip the brass screen on the inlet site. Seal back with ribbon dope.

If this looks ok, remove the 4 bolts holding the head on. Keep all of the sandwiched plates together in order so they go back in easliy. The cylinder sleeve will come off with this stack. Look at the plastic surround on the piston and the inside of the cylinder. In my case, this was eaten up from too many hot summers and cold winters in the garage. The rubber/plastic had hardened and eaten part of itself away. To replace the entire piston assembly on the WL651004AJ, you'll need to get part number WL211000AJ, which in some books is superseded by WL211000SJ.
If the cylinder is scored (meaning that by scraping your fingernail against the lines in it, you catch the nail) then you're probably better off replacing the whole cylinder unit.

when i turn it on motor runs but it will not build up air pressure

Possibly the pressure switch needs to be adjusted to reach a higher pressure. Trace the power cord into a box that also is piped into the pressure line from the compressor. If the pressure works itself back down again, use thread-locker fluid on the adjustment screw to hold it's setting.

This is a simple way to replace your belt without the old one for comparison. Use a piece of rope thick enough to fit inside the pulley, (cloths line works well). Simple wind it around the pulleys marking where the two ends meet. Cut and measure it. Take the piece of rope and measurement to a auto parts store and ask them to match it up. An automotive belt will work fine.
If this is a flat belt they should still be able to help you.