Filteration System

[Suppose to be summer fun]

OK, we have an inground pool that's about 20 years old. For some reason we cannot get enough pressure through the lines to properly filter the pool water. We keep losing water because of a leak from the main drain, so the water is not up to the skimmer box to put it on 1/2 and 1/2...so, we have it on full and filter, but again, we're not getting enough pressure.

We have already repaired all leaks that were new this year in the pipes, but still do not know why we are not getting enough pressure.

My husband thinks that we do not have enough water in our sand filter....??? Could that be it? I know that the sand filter recommended 1/2 the tank full of water before filling to the mid point with sand....but I wasn't around when that was done??? Please help...I need a tan.