Well I have a slide valve, not a multi-port, but I can't be 100% sure on the valve's condition. (It does seem to run normally in the run position, and backwash normally in the backwash position...)

I did open the tank to have a look at the screens, and couldn't see any holes (I didn't lift them out).

On a side note the air valve attached to the preassure gauge is locked (rusted) into the closed position, could this be the problem?