Here is a Shurflo pump replacement chart (part numbers)
https://www.depcopump.com/catalogs/o...log110/158.pdf
If replacing the pump a Shurflo 4048 would be good replacement
Constant pressure but uses a Bypass to control low demand cycling
If adding an in-line check valve it should be installed in the suction line so pump body stays full & pressurized
*if discharge side a leaking pump check valve will allow pump body to drain back which cause pressure switch to close and pump cycles on/off