last year was my first year with a pool. Which I bought used, spent the last season with 15-20 psi at the filter. Thinking the sand needed replaced I changed it today with no change in pressure. Its around the same pressure when back flushing, yet I can make the gauge pressure change by using my hand to apply different amounts of "blockage" at the return. and the gauge will go to zero when off. Could the problem be in the valving of the filter?