My air compressor will not fill to maximum capacity, why?

You didn't say if it contiues to run or if it shuts off at 20#.

If it shuts off at 20# adjust the regulator, if it continues to run check to see if the drain spigot is closed all the way and check for leaks..

Yes you can convert it, however it is designed to compress a refrigerant, not air. This makes it inefficient as well as slow. Also, you have to remove the refrigerant at a collection site. The amount of money this would cost you would be a lot more than it would to visit your home repair store and buy an air compressor with an eight gallon tank with all the fittings and hoses included.

Bad air pressure shut off switch, would be located at the pipe comming out of the tank,

there is an adjustment, but it could be bad,

turn up the air pressure regulator. Should be located near the gauge. Turn clockwise

locate and turn up the regulator , usally found on the side of the pressure gauge

try the adjuster valve. you might have busted seals also