Spa Check Valve Pulsates and Spa Drains Into Pool Overnight

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So I have assembled all the parts necessary to complete this task but I have a question before I cut off the pipe (spa return) where the red rectangle is.

I have an elevated spa. When I cut this pipe in order to install the check valve, will water pour out of it until the level in the spa and the level in the pipe are equal? Should I try and drain the spa down as much as possible before I cut? I obviously need to glue the check valve in and I can't do that if that pipe is full of water.
Yes, drain the spa down before cutting the pipe.