Regulator on mac5200

It is quite normal for the output line to exhaust into the atmosphere until the new pressure is achieved when reducing pressure. If it continues to leak the regulator is faulty.

https://www.helpowl.com/p/Makita/MAC5200/179201
This site will help you.

An M9 x 1.0 should do the job for you.

So you are saying that at 140 it leaks down to 120 and then it is fine? You need to check for leaks with soapy water and listen closely to the exhause and intake valves when it is off. Check the unloader valve and the regulator assembly.;

problem is most likely not the regulator but faulty reed / poppet valves in the compressor heads
the described pressure is about where the valve stop the air returning for the piston strokes
as for repairing or junking, it will be cheaper to repair if the parts are available

Plunger inside the regulator could be stuck or adjustment knob stripped. Let the air out of the tank and take the regulator apart, lube with light grease (no kit available for this reg.). The factory part is expensive so replace (if needed) with generic, careful with size however because there is not much room under the panel and between the gauges . Easy temporary fix. Remove the safety pop valve and install a tee (or check under the pressure switch for spare port) re- install pop valve on the tee fitting. On the second port of tee fitting install a quick connect valve for full pressure. Generic regulator is sold at Lowes that can be installed on the temporary port.

Its likely an o ring inside the valve asembely one of the knobs has an air bleed to regulate the amount of pressure between the tank and the oressure that you set to come out the hose .You should be able to disasemble by eighter unsrewing the valve all the way out or removeing the knob and loosening the nut that holds it in and replaceing the o rings that you should be able to match up at a hardware store

Chances are that the pressure switch, were you turn the compressor on and off, is bad. When they go out, they no longer build pressure to the regulated level you are looking for. If you haven't already, try turning up the regulator, if no more than 100psi build's, then it's a sure thing, bad pressure switch.

There are two types of regulators used. One has a rubber diaphram sealed at perimeter with body and a piston type. Most likely you have the piston type. Remove body containing piston/ spring mech. Examine piston lip seal. Lip will face toward body of reg. and must fit snugly into cylinder. Make sure cylinder is smooth and is well lubed.
If seal/piston fits loose remove seal and wrap some teflon tape in groove and re-install lip seal /test for snug fit. Notice small metal actuator extending from center of reg. , if it is circular (about 1/8 dia) then there should be an oring of the same size mated/fit into the piston. If this oring is missing it will leak as you describe. Lastly, there should be a gasket or oring sealing outer shell to body. If you have diaphragm type look for holes, tear or shrink too small. Parts are at www.toolpartsdirect.com or Lowes in the plumbing department good place for orings and silicone lube. Good luck

piston rings or valves in the head,,, valves are the most likely one.