This definitely sounds like a problem with the load/unload system.

So it's either the inlet valve, the solenoid valve(s) or the electrical connections/cables/controller.

Since you already changed the solenoid valves, that part should be OK. Did you only change the electrical coil, or did you change it completely?

It could be something as simple as a loose connection or a broken cable, or a bad contact in the controller.

To find out, trace the system back from the controller, to the solenoids, to the inlet valve.

Try to press load/unload on your controller and while you do, check if the solenoids are switching (you can hear a click).

If there's no 'click', or it's too noisy to hear it, get a multi-meter and check for a voltage across the solenoid. Press load/unload and see what happens.

If there's no voltage coming to the solenoid, check the connection, cables, etc.

You can also check with a screwdriver. The solenoid becomes magnetic when activated.

If there is, and the solenoid is new, you can assume it works. If the solenoid is old, it can be dirty inside and the dirt blocks the control air.

A test you can do is disconnect the line going to the inlet valve and see if control air comes out when you load/unload the machine.

(be careful, this eliminates the control functions and might cause the compressor to built up pressure without control until the safety blows)

If all this is OK, then the inlet valve simply stays closed. Probably it's dirty. You can open it up, clean it and replace al o-rings and seals. Be careful again. There's a very powerful spring in most inlet valves. Make sure you know what you are doing, otherwise call in some experts.

Good luck and love to hear how it works out!

Cas