check valve help

[rodnie74]

My 12 000 pool got a spa and everytime the pump turn off the spa water level goes down to pool water level, i read that bad check valve cause this i buy a new one i want to replace so i buy 2 couplings and a peaice of pipe cause this check valve are the one you use with glue, a sealed one , my question is if i need to close returns or drains valves before cut the pipe or can i cut the pipe without close nothing, i want to know first cause i dont want see all my yard flooded any help or tip thanks