If the Pearson Pump is not building pressure, especially directly after a rebuild/service, more often than not the valves were not properly reset. Any time the Pearson Pump is opened for service, it is imperative to reset these valves to the proper position.
Note that the Pearson Pump is designed to receive a seal kit service at 2,500hr intervals, and a full rebuild at 10,000hrs.
Please see below for a guide on resetting the valves from the seal replacement manual:
To attempt to remedy this without opening the Pearson Pump, when the system is running, kink the brine discharge hose for 1 second and immediately release, up to 10 times in succession (though generally only 1-3 times is needed). Do not kink for longer than 1 second or hoses can blow off/internal damage could potentially occur.
This injection of high pressure can often reset stuck or improperly set valves to the desired position.