I'm using a Wilwood adj prop valve and am getting a leak in the 1/8" pipe fitting out of the valve. Replaced the fitting with a new one and same thing. Should I be using and type of thread sealant. The fittings that came with it had some type of sealant on them, like thread locker.

That should be a flare fitting.

I think its a ground joint like a flare but no sealent is needed.Look inside the valve and see if it is not damaged.I is almost like a seat on a faucet it needs to be smooth.

My adj prop valve is SSBC, and the flare adapter that screws into the valve itself is 1/8 pipe thread. I used some teflon thread sealing paste there. No sealant needed on the "flare" side of the fitting.

BPOS, thank you. Exactly what I was looking for.