As said if you dead head the pump and it starts laboring your issue is leaks in the woods. Could be squirrel chews, taps popped loose, saddles leaking. Those diaphragm pumps can not over come much for leaks. The Delaval pump should be able to handle much more leaks as it would have substantially more cfm. If you have multiple shut offs you can isolate areas where the biggest leaks are.