A picture is worth more than a few words. The valve in question screws into the discharge "snout" of the pump.



Sherman, my experience with the FOT pump has duplicated yours - unfortunately, I do not believe you have the hydraulic room to add this part (it adds ~1.1" to pump length) but you would know better than I.

On Isomac and similar machines
one valve is an air breeder valve
one is a one way valve (also called check vale)
then the OverPressureValve as well

I had the same problem. No water at the grouphead. The machine would heat up and whistle with steam. But no flow of water.

Oddly enough I tried several times to determine the problem. I thought the pump was primed. And still, no water.

I removed the pump from the harness, disconnected the hose to the reservoir and fed the unit water through a small cup. I could clearly hear air in the pump, so I tried running water through. I couldn't get any water from the cup to get sucked into the pump.

I tried this several times. Several days. And today it worked. I did nothing different. So my advice, try try again. If there's any air in the line, it might be causing the issue.

Good luck.