On my sprinkler system I installed, about 4 months ago, a system that siphons, from a large tank, a mixture of fertilizer and iron stain inhibitor. The system works fine but it had to be installed by drilling a hole in the pump inlet and using a 3/4" pipe tap which is where the check valve for the siphoning system hand-screws in with some thread tape. There were no leaks initially but this morning I noticed a small pool under the pump and saw immediately that it was dripping from where the check valve was screwed in. What is the best way to seal this leak? I tried wrapping the plumbers tape around thicker but it didn't help. I don't want to cement/epoxy it in, I want to be able to remove it if I need to, but I would be fine with something semi-permanent like caulk. Would caulk work well for this?
Tighten the joint slightly.
Pipe threads are tapered - the PTFE thread tape (or pipe dope - I'm currently more of a fan of PTFE pipe dope than of PTFE thread tape, though I used to be a big fan of the tape as opposed to old-style dope) provides a seal for the "helical leak path" (the fact that the female threads are a bit sharper than the top of the male threads) and some lubricity for easier tightening.
A slow drip type leak is often just a 1/4 turn from not leaking.
If you take it apart, you need to clean off the remains of the old tape and apply new tape (one reason I now prefer the modern dope - you can just add a bit more, with no need to clean off the old, and you don't have all the tape shreds to deal with.) Excessive thickness of tape won't help the seal - more is not better.