Bleeds off air as it builds up pressure, causing longer run time. Sometimes it continues to bleed air pressure after tank reaches maxime pressure then not letting motor to restart

SOURCE: I have a Porter Cable

Replace the regulator

SOURCE: I have a C2006 Type

first you have a reciprocating pump and the hissing you ear probably comes from the unloading valve on the pressure switch(it will look like a bike tire valve)if there is air coming out the valve when the compressor is off it probably means you have a bad check valve.This valve is use to unload the pressure inside the pump for an easy start .a motor always drive more amps on start up if you cant unload this pressure you will trip the overload(you probably have a reset button on your motor)check the specs on your pressure relief valve ,its a small pump maybe its just adjusted to high.in this case your pressure relief valve will open .If the pressure is fine and there is air coming out the relief valve then you have to replace it!to adjust your pressure turn the screw on your pressure switch clockwise to increase and counterclockwise to decrease .usually you have a 30 psi between cut-in and cut-off called the pressure differential.It can be adjusted on some pressure switch.be sure you put some teflon tape on the treads if you replace something to avoid air leaks.hopes this help!

SOURCE: My four-year old PC pancake compressor starts fine

First take off the head on the air pump and check the reed valves on the valve plate. If they got hot and warped or cracked they'll work to a point then no more pressure can be built up. Next check the pistion ring and cylinder wall. If either is damaged you'll only get so much pressure build-up then it will leak past the piston.

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SOURCE: My porter cable C2002 Type

I have the same unit and had the same problem Its a good little mechine but the problem is the houseings for the gauges when you are loading the mechine can get hit and move up and this will bend the neddle on the gauge and make it hang up . You can carefuly remove the lense with a small screwdriver it just snaps on and off then gentaly bend the neddle pointer backand snap the lense cover back on and it should work again . Just be carefuly when you transport the mechine not to let the base of the gauges touch anything