Use Non-hardening, PVC compatible thread sealant instead of teflon tape. Oatey Great Blue Pipe Joint compound is one. Too much teflon tape can actually make it worse.

You can run with leaks - take note of what it's leaking onto.

The union fitting, it's the type with an o-ring? make sure the o-ring, the channel it sits in and the flat surface it mates with are all clean and intact. Lube the o-ring with silicone o-ring lube (be careful, really sticky and messy) and re-make the union. Be sure to line it up good before tightening the collar.

Good luck