Flojet Pressure Controlled Pump 4405

[nigel1]

Good day,

Hoping someone here has a good idea on how these pumps work.
Have one of these fitted on the fresh water system. Pump has a pressure switch which turns the pump on and off as required.
A few days ago, the pump started to cut in when no water demand was made. I had a spare pump, and swapped out the pressure switches, which was my first guess at the problem. That did not resolve the problem, so swapped out the whole pump, and problem gone.
I overhauled the original pump a year ago, all internal valves and diaphragms were replaced.
The only part not included in the spares kit was the Bypass valve. This is not even shown in the parts list (see:
Flojet R4405143A - Pressure-controlled pump with internal bypass / Flojet Water System Pumps / Flojet Marine Products / Xylem JabscoShop - Jabsco & Rule Pumps and more - from the experts
)

On the top of the pump next to the pressure switch is a cover held in place with two screws, and under that, a spring loaded valve.
Does anyone know if this is the bypass valve.
When I removed the rubber valve, there was some crud under the seating face.
Also, anyone know how this bypass valve works.
Seems odd that this part is not included in the spares list.