My Porter-Cable Air Compressor is drawing too many amps
At 15A, that is a 3HP motor (assuming 240V single phase here), the "6HP" is a marketing term that means WHEN the motor is slowed down by a change in the load to it's "Break Down Torque" speed, it will develop 220% of rated torque, VERY briefly, in attempting to get the motor back to speed, so 220% torque at a just slightly lower speed results in 200% HP for that brief moment, and they are allowed to present it that way (which I think is a shame). the operative issue however is that during that moment, which for a compressor will be when the pressure is highest, that motor can actually be pulling 33A, not 15A. Most likely your meter is averaging and/or too slow (not enough samples) to be giving you the correct current at the moment the breaker is tripping, but of course, the thermal element on the breaker is seeing and reacting to it. That would also explain why you can't reset it when the pressure drops, because at that point it has nothing to do with pressure any more, it's a thermal cool-down time frame and thoise are specifically designed around the time it will take the rest of the equipment to cool down too.

Changing the caps will not fix this. You need a bigger circuit. a 20A breaker can only be used at 16A continuously (80%). Upgrade to a 30A breaker and #10 wire.