if there is no fluid moving from the manifold connection to the shuttle valve, there can be only two things wrong. either the pump has failed, or the end of the pump motor is not engaging the key way of the pump. the adapter may be missing or broken.
the only way you are going to find out which it is to do as doug suggested, and remove the pump motor.
you may be able to locate this part, but i wouldnt hold out any hope. the new pump assemblies do not use this adapter.
hwh is not stumped, its just that you need to do the diagnosis before spending that kind of money. all hwh can do over the phone is SUGGEST a solution.
only a hands on diagnosis will come up with the solution.
bty, when adjusting pump pressure, a 5000 lb pressure gauge should be used to get the correct pressure. harbor freight sells one for about 10 dollars. its fairly accurate. ive checked against some that were certified.
