For 4 years I have aerated my 1 acre pond with easypro scr50 1/2hp pump with 400ft 5/8” poly line going to my pond then at the pond I have a splitter to two double headed diffusers. The line from the splitter is 1/2” weighted for about 60 ft to each diffuser. My problem is the after 4 yrs I have replaced the gaskets, cylinders of the pump because the pressure dropped to the point I was getting nothing from the diffusers. I also cleaned the 4 diffusers at this time. I have walked the entire length of the line from the pump to the splitter checking for air leaks. I have checked the check valves on the diffuser plate and all looks great. I can put the diffusers in 2 ft water and they bubble away but when I put them at 6 ft water I get nothing. The PSI at the pump is about 20 PSI which is suppose to be 3.7CFM. I am getting what I believe is great air pressure at the splitter. Any ideas why I get nothing at 7ft but get 2ft diffusers bubble away?

Double check the gauge, make sure that it is reading right and not stuck on 20 psi when the system is off. Did you also take the piston apart and replace the gasket material that acts like rings in a gas motor?

The only difference between the 2' depth and the 7' depth is the backpressure. I'd say roughly 1-2.5 psi backpressure at 2' depth and 4.5 or so at the 7' depth.