I broke my oil fill cap on 3HP 125 Psi compressor. What can I use to replace fill cap?

SOURCE: the compressor doesn't build up any pressure.

AIR FILTER TOTALLY CLOGGED. BROKEN PISTON ROD. MORE LIKELY IT HAS BAD VALVES. BROKEN AND NOT CLOSING ALL THE WAY. PROBABLY HAS A VALVE PLATE UNDER THE HEAD THAT CAN BE REBUILT OR REPLACED. HOPE THIS HELPS. JODY

SOURCE: compressor shuts off before reaching max pressure

Yes,but if the max is 125 you don't want to increase it above 110 and set off the pressure relief valve.

SOURCE: Craftsman air compressor 1 HP 12 gal 125 psi

this is probably an "oilless" compressor, it has "piston" with a thin ring of leatherlike material acting as a piston ring, the "cylinder" is an aluminum sleeve - what usually happens is the cylinder gets scratched from dirt or just wear or the leather ring deteriorates, that's why it will go only so far - the only fix for this is to replace the cylinder and the piston

SOURCE: Air compressor won't go over 40 psi?

my ace air compressor which is 26 gallon 5 hp will not get over 60psi what is wrong with it please?

SOURCE: My compressor will not go

The problem with compressors not building pressure is usually a bad pressure switch or bad regulator. Does the motor shut off at 20psi or does it continue running and not build pressure? If the motor continues to run with no increase in pressure, the you should go over the machine looking for air leaks from your regulator back all the way to the motor. Any air leak while the compressor is running can cause the pressure to "peak" way before the intended shutoff that it is regulated to.