A Pool Cartridge Filter has an O-ring between the filter head and the filter body. If you see water leaking or spraying out at this junction, it's time to replace this O-ring. This guide shows you how to replace the O-ring on a typical pool cartridge filter. Note: We'll be using a D.E. filter for this guide.
Next you will have to relieve the pressure that has built up in the pool filter. Note the pressure on the pressure gauge. It will typically read between 15 - 20 psi. To relieve pressure turn the air relief valve on top of the filter counter clockwise. Water will spray out initially as you see the pressure on the pressure gauge go to 0 psi.