So he used glue in valves? Jerk! I'm assuming you mean a ball valve, not the filter's 6 way valve.

I would bite the bullet and replace the ball valve with one with threads on the inside. Then you glue a threaded nipple onto fresh PVC and do what you need. They also sell double-male fittings, both long and short in 1.5" lengths. That means it's already threaded at both ends and you don't have to make your own.