Dewalt D55155 compressor problem: after reaching max fill (120psi) bleeds/looses air @ the on/off switch (out of hose comes off regulator. If regulator set @ 60psi it will bleed till 60psi.

SOURCE: dewalt D55155 air compressor regulator installation

The d55155 unit suffers from regulator problems due to weakness in the design of the manifold. The manifold parts are very light duty metal and will break during installation if not carefull. If you can return the parts to dewalt it may be more efficient to to use a generic pressure regulator from www.grainger.com for about 20 bucks and mount using a couple of 1/4" tees directly off the pressure switch. Remove the hose from the pressure switch that feeds air to the manafold . Install parts so that the quick disconnects, guages and new regulator are accessable from the side. It will not be pretty but problem will be solved for long term. We repair other brands that come with control panels using the same method. Pressure regulators built into a manafold will fail. For demanding customers we have recreated the manafold using 1/4 pipe fittings and refit the gauges and grainger regulator into panel.(lots of work) But leave the safety unloader at the pressure switch. If all you need is a good functioning compressor same time and money and mount to side. Good Luck and email if you have questions or other issues with your dewalt.

SOURCE: Makita MAC2200 runs fine (with new Manifold Switch

I have worked on many makita compressors and have run-across this problem on a few of them. Your compressor has a check valve that is supposed to hold tank pressure after the switch reaches set pressure. There is an unloader valve built into the pressure switch that releases the pressure in the discharge hose after the unit shuts off. The unloader allows the compressor to restart with-out any pressure in the discharge line. If the brass check valve is leaking pressure back into the system, air will leak out of the unloader valve. Follow the discharge line to the tank and you will see the brass check valve. To test valve for fault, run the compressor till it builds around 80 psi and unplug the unit from the wall outlet. Remove the 1/4" hose connector from the check valve and check for air leaking out of the valve. If it is leaking you can order form www.toolpartsdirect.com. or call the makita 1-800 number. The price is about the same. Good Luck on your repair.

SOURCE: Dewalt 4 gal air compressor will not build pressure in take.

The bleed valve is an unloader of sorts that allows the compressor to start with-out a load. It closes when the pressure in the airline reaches about 20-25lbs with good volume. If air volume is not sufficient, the valve will not close. To solve the problem lets do a few tests. A long cord will reduce power and reduce air volume. Try powering the compressor with-out extension cord. If bleed valve closes when you do not use a cord, the compressor is not getting enough voltage, use shorter cord of the 12 gauge quality and more hose to reach work. If valve still does not close, check for air leaks on all fittings including around pressure switch and drain valve with soapy water. Repair as necessary and retest. If no leaks were found, temporarily plug bleed valve port with screw/bolt and run compressor till set pressure cut-off while counting minuites/seconds. The compressor should fill and shut off within three minutes or so. If it takes an unusually long time to fill, then the reed valves /valve plate is worn and leaking pressure back into cylinger. Leaking valves will usually result in very hot discharge airline/air. Reinstall the bleed valve after test. Parts and partslist are available at www.toolpartsdirect.com . Good luck on the repair and email if you have other questions.

SOURCE: I have a dewalt D55155 compressor that has not

have you turned up the regulator, the big black know by the output fitting

You may have to open andclose it a few times cuz it may have trash or be stuck

SOURCE: I have a Bostitch CAP2000P-OF 6 gal compressor.

Something is wrong with your "unloader" I should think. That is the device that removes the pressure from the cylinder so the machine does not have to start up against its full air pressure
There will be a narrow air pipe connecting the compressor to the switch assembly which bleeds off the air when the power is off. That hiss you get when you turn it off or it turns itself off.
I think you will find a leak there or possibly the non return valve that stops all the air escaping when the unloader valve is open has failed or got a foreign body stuck in its seating.