Dresser series 400 unloader valve plumbing ???

by Scott
(Washington state)

I just bought a used dresser series 400 air compressor and Im not sure how to hook up the unloader valve plumbing...

right now it has the two unloader valves one in each head plumbed into a t and then plumbed down into the top of what I understand is the oil pump/unloader valve diaphragm ....

and in that square diaphragm box there is one threaded fitting that goes out the front and one that goes out the bottom with nothing on them??

anyone know where these two fittings should be plumbed to?

Right now the unloaders do nothing.

Not a very good pic, will upload more after work
Actually, no photo came in with this post.


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May 06, 2017
Constant pressure?
by: Anonymous

I just put together a rebuilt compressor. After it shuts off, it is leaking air ( Sorry, this comment is off topic. We would like to answer your question, but need it to be in the correct forum. If in doubt as to where it goes, start a new post in the ASK forum, and we'll move your question to where it needs to be and try to get you some answers. - Moderator )

Apr 20, 2014
LeROI, Dresser, Westinghouse, CompAIR unloader plumbing
by: Anonymous

Basically, out the front would be a connection directly to the tank or air receiver.

When there is low oil pressure in the pump, as when the pump starts up, the air pressure from the tank goes through that little square box(which is a check valve) and pushes down on the cylinder head unloaders and no air is pumped.

Once oil pressure builds up, the check valve is closed, the cylinder head unloaders close and the pump starts pumping air.

The connection at the bottom is used in constant speed operation and would go to an unloader pilot valve. The pilot valve would open at the high setting allowing air to go to the cylinder head unloaders and opening them.

When the pressure in the tank drops to the low setting of the pilot valve, the valve closes, stops air flow to the head unloaders, and the pump starts compressing air again.

This port could be plugged or left open for regular on-off, pressure switch operation.

I hope this helps some.

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