Air leak coming form the pressure switch - Dewalt 2 Hp 4 GAL Air Compressor

Some pressure switches have a valve that reliefs the compressor head pressure when the motor stops and closes the head pressure relief valve when the motor runs.
There is a check valve that keeps air pressure from coming back to the compressor head and the relief valve.
I think the check valve is leaking tank pressure to the relief valve and compressor head.
The check valve is where the pressure switch mounts on to and it mounts threaded on to the compressor tank.
Try replacing the check valve

The air is coming out of the pressure switch but the problem is the tank check valve. The tank check valve is on the tank where the large line from the pump feeds into the tank. There is also a smaller line, either plastic or copper, from the check valve to the pressure switch. When the pump shuts off the check valve is supposed to close and the pressure switch opens a valve that lets the head pressure bleed off through the small line. This lets the motor start the next time without any pressure in the head. If the check valve isn't closing properly the air will leak out as you describe.

If air continues to leak from the pressure switch after cycle is indication that the check valve is not sealing or the pressure switch diaphragm has failed. Easy fix. Follow the 1/4 inch airline to the tank and remove the line from the brass valve(check valve). If air is leaking at the brass valve, remove it and attempt to clear debris under valve or replace as needed. If no air is leaking from the checkvalve, then most likely diaphragm is leaking. You could replace the diaprhagm or the whole switch. Generic switch is available thru ebay about 18- $20

This sounds like the compression release valve is leaking. This valve bleeds the cylinder pressure to allow the motor to start without having the load on it. On this size compressor the valve is often built into the pressure switch. It is a little "pintle" type valve and you will hear hissing while unit is running but the opressure will not build up very much. When the pressure switch calls for the unit to run, the little valve closes with the switch. COntamination or other stiction keeping the valve form closing will cause the problem. You will find this near a tube that goes to the pressure switch. The switch itself hooks into the tank, but this little tube will connect to the cylinder head or the main tube from the compressor. Yours may look similar to ones at this site:
http://www.generalairproducts.com/Fire-Protection-Products/accessories-pressure-switch-a-switches-check-drain-a-pressure-relief-valves.html
The valve is also commonly called the "unloader" valve.

replace your tank check valve..the noise you are hearing is coming from a bleeder valve that's bleeding air pressure from the head so when it starts its not starting up against pressure

Hi, my name is John. Welcome to FixYa. Will it come on at all? Check the outlet circuit breaker, if it comes on at all, check where the line comes form the pump to the tank, there got to be a leak, you may have to remove it in order to check under it. Check for a reset button next to the wire coming to the switch. Thanks for choosing FixYa...John

There is no unloader on a 2340N5 compressor. However there is a pressure switch that cuts the motor on an off. Located inside of the pressure switch is an air bleeder. When the pressure switch is opened the motor is shut off. A small lever in the pressure switch pushes down on the bleeder valve which lets the air on the compressor head to be released. This allows the the compressor to start up freely. There should be some air heard bleeding off at the pressure switch. If air continues to bleed off all the time the tank check valve is leaking.

If air is leaking back thru the compressor an air leak sound coming from around the air filter (intake) you have a valve leaking allowing trapped pressure to escape thru the air filter (Intake).

I would search the web for a flow chart. I have not seen one.

follow the line backwards down to the block and clean the breather(if fitted) this is not an oil leak, the pipe is there to **** up the engine pressure when decelerating.........the air filter should be changed every 20 thousand miles/2 years.....

There is a check valve where the line goes into the tank. The purpose of this valve is to keep air from coming out of the input line. This valve is likely the culprit and it is leaking out of the unloader. The unloader is actuated by the electrical switch when the compressor shuts off, and it relieves the pressure on the line so that the pump is not under load when it tries to start.

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.