Actually, I'm surprised that the internal check valve lasted as long as it did on your unit, as the one on mine lasted all of two weeks before failing.
This is a silly question/remark but I'll ask anyway. The internal check valves on these plumps are failing in the open positions correct.
Chris Miles
For Better or Worse I own a DeLorean!
1983 Grey Manual, VIN #16409, Fresno, California