>Looks like it may be some kind of blockage in the system probably at the
>check valve.

There is a limit switch at each end of the rack, I THINK it is to ease off 
the pump pressure when you reach full lock. If the electrical connections 
were wonky on one/both of these then that may cause the pump/pressure to 
turn on/off sporadically and randomly. Or am I just p*ssing in the wind?  tim s.