When my Fleck 2510 starts to back wash, the water is really hissing through it initially. However, after 1 or 1.5 minutes, the rate slows down. I don't understand why the rate drops because I can stare at the piston and I can see it hasn't moved and "the wheel" is still on the same group of pins. I was just curious what causes the back wash rate to slow? Theoretically, I'm supposed to backwash Centaur Carbon at 6 GPM in a 10x54 filter. Is that rate based on the initial rate that I get for 1 to 1.5 minutes, or is that based on the subsequent rate?