Why won't my air compressor pump up over 40 lbs.

There are a lot of reasons it might not. But if you want more details WE need more details.

There are a lot of reasons why it might not. But if you want more details WE need more details.

It runs fine but it will not pump up more than 40lbs. It continues to run but will not gain any more pressure. Could it be the pressure switch?

Where is this 40 psi measured at... the tank, or after the pressure regulator?

If its an electric compressor, the pressure switch shuts off the compressor,you state that isn't happening it keeps running, if it's a gasolene or diesel powered compressor, that switch would be an unloader and could pontientually cause such a problem. IF that 40 psi was a tank pressure, you either have a healthy leak you would hear, or the compressor is worn out and it can't provide more do to worn components. IF this is measured at the line after a regulator... someone turned it down, simply readjust the regulator.