Low water flow when running backwash/rinse/waste

[lizabetha2006]

My pump appears as if it is working fine in filter mode. Suction is good, and the water flow coming out of the return is good. When I need to backwash though I have good pressure and suction for 1-2 seconds then suddenly the water flow drops to about half or a third, the pressure gauge jumps up in the yellow, and there is very little suction from either of the two filter ports in the pool (I'm not sure what they are called). After running for about 30 seconds I get the Error 90 code on the pump, which is the low water flow error. I also get this error if I run the pump in rinse or waste mode, however, the suction and water flow is slightly increased in these modes.

My pool does not have a skimmer built in but I have attached one of the over the side skimmer baskets. I have checked to make sure that the skimmer basket isn't clogged. I have taken the plastic grids off of both of the suction ports to make sure they are not clogged. Emptied the pump basket. I have checked to make sure there are no water leaks anywhere.

This is a new pump, that I just purchased because I thought my old pump was broken since it was doing the exact same thing. A bit of history about the old pump, It worked beautifully for about 1.5 years. I put floc in the pool and rookie mistake vacuumed to the filter instead of waste. The pool pump started acting up directly after this happened. I purchased a new pump and new sand. Installed everything and the new pump immediately does the same thing the old one did. This leads me to believe that the issue is before the pump, but I don't know what could be causing it, as far as the pipes from the pool to the pump, everything seems fine.

Any thoughts?

[Teapot1]

Sounds like air getting in, with the faster flow of the other settings (not filter) more chance of the pump drawing faster and harder so exposing an air leak. That is provided the pool is full, dont laugh I have been called out to that issue. Check all fittings and O rings on the suction side and change any with a flat (compressed) face.

[lizabetha2006]

I purchased new parts for everything before the pump. Replaced the hoses from the dual ports to the pump. Replaced the washers and o-rings and replaced the plunger valve. Made sure everything was tightly put together. I am still having the same issue. Any other suggestions?

[Teapot1]

You can do all that and still have an air leak. The symptom you state is normal for an air leak so you will have to look harder I'm affraid. People use different ways of trying to locate these leaks from spraying a bit of shaving foam on to the unions and see if it gets drawn in or reduces the issue. Even a tiny crack in the pump prefilter lid or motor casing can cause the problem, it why I prefer to position the pump below the pool water level.