Check valves look fine. It's an industrial pressure washer. 7.5hp, 240v, 3phase motor and diesel burner. Previous owner reports not used in 10 years, but functioned fine then. Any ideas? I have 3 phase power, and the electric motor runs fine. The motor spins, but the pressure doesn't rise. I tried it 6 days ago. Only for 5 to 10 seconds, not to ruin the pump. Today, the pump check valves were dry, as well as the visible internal parts of the pumps I could see.

let me start with a description of the pool.1 main pool 4ft. to 8ft. deep and pool 2 is 1ft. to 3ft. deep. the water level of pool 1 is 18 inches higher than pool 2 and that pool 1 overflows to pool 2 on the side they are connected. we have the hayward DE7020 filter with a vari-flo control valve SP-715XR50.do i need to put check valves for the drain system right before the pump suction or after the vari-flo valve for the return system or both. do i really need to install check valves even if all pipes have ball-valves? where will be the proper position for them? thanks

individual station, or are centrifugal sewage pumps designed to withstand back flows without a check valve?

The supplier says they are used all the time and should be ok. The plumber is hesitant. If I don't use them I have to spend more money buying other fixtures. Here is what I have: one thermostatic mixer with volume control and an one-input-three-output diverter, one shower head and one hand-held shower. The goal is to be able to use the shower head and the hand-held independently or at the same time. The original design was to have the diverter to feed the shower head, when set to 1st position, feed the hand-held shower when in 2nd position, and feed both when in 3rd. But for the last situation to work, we have to t-off the main pipes of the showers which will cause the water to go back to other shower when we intend to only use one of them. Basically the problem is if we don't use the check valves we would have waters in both showers at all times.