I unplugged the mono-valve solenoid and at first did not get any heat so I tapped on it with a small ball peen hammer. I then got heat out the vents!

So, I removed the solenoid and took it apart. The piston had some rust so I cleaned it up and put it all back together. It now moves smoothly but I don't think the system is working right yet.

It was a warm day (76F) here so I'm not sure if should get hot air out the vents or not but it felt like ambient air out the vents UNLESS I turned the temp wheel all the way to the detent. I tried a second pushbutton controller to make sure that wasn't the problem. I tried a second solenoid that I could plug in and watch. Unless the temp wheel was on the detent, the solenoid was engaged.

So...is there anything else besides the push button controller that signals the mono-valve solenoid? I will check tomorrow morning to see if I get heat but it sure seems like I'm not getting heat unless the the temp wheel is on the hot detent.