It seems pretty clear that your problem was caused by the pressure switch
failing to open and the safety relief valve opening just like it was
supposed to. A faulty check valve cannot cause this problem. It can cause
the unloader to let all the air out of the tank when the pressure switch
trips to shut off the pump. The pressure switch needs to be cleaned,
adjusted, or replaced.
Don Young
"Dave, I can't do that" wrote in message
oups.com...
So, where does your air escape from?

I am not sure. By the time I had unplugged it and dragged it out from
under the lathe, it had reset. I was able to feel that it was coming
from around the switch box. I turned it back on but it operated
normally. Hindsight being what it is, I should have dragged it out
before unplugging. sigh

I'm with you about the valve. I have just checked CH troubleshooting
guide on their site and it suggests the check valve needs replacing. I
can't find my model number there so have left a message to see what's
what. I bought this in about '98 so they may no longer support it for
parts. I'll be really ****ed off if that's the case.

vbg Sears HP I am guessing. I think I did a calc some time back and
it was about 2.5hp based on the Watts rating. Still a lot of power to
eat if it was running all night.

Where did you buy the new check valve? I might just do that if CH no
longer support this model. The unloader worked fine when I switched it
back on and but very unlikely it was the cause.

Dave