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Ok, well one thing to check is to make sure that the control linkage is actually turning the control shaft in the pump. Once it is verified that the control is ok. The pump is really not serviceable in the average shop. It is generally more cost effective to send to a hydro rebuild shop. Just simply remove the pump and send it on its way. Usually they tend to destroy internal components when they quit pumping and will need extensive overhaul. I also recommend flushing the motor as any contamination coming out of the pump will end up going through the motor and potentially ruining it as well as your freshly rebuilt pump.
It is possible to rebuild the unit in a well equipped shop but I have found that usually it is just better to send them off.
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