Tools, Sharpening and Power Machinery - AIr compressor struggling

A while ago I posted my Sears Air Compressor Model# 919176830 was acting this way: I took a new 2 hp (rated to take 3 hp models) motor and made a mounting plate to take place of the old sears motor that didn't work anymore. I turned it on and it filled up to 20 psi and then the motor struggled to continue and started burning the belt. My thought was that it had sufficient horsepaowed but he mottor was trying to overcome the compressor for some reason. You guys asked me to check the motor. I checked both the motor and the compressor. I took the compressor check valve and replaced it as well as inspected the heads and valves. All was well. I brought the motor up to a motor repair and ten dollars later he hooked it up, read the voltage, amps and it all checked out ok. The motor was definitly struggling to work the compressor and was doing it's job fine. It was pulling 24 amps right before it stopped trying. So...my only other option is to take the compressor apart until I get to the bottom end of the cylinder to inspect, but if it will make air compress up to 20 or 40 psi - to me it works and actually does it's job of compressing air. Why would the dynamics change after 20 or 40 psi?

Crazy,
Ryan