Nil writes:

On the other hand, the elbow to water valve connection is metal to
metal, and should have Teflon tape. It doesn't look like you've
screwed the elbow in very tight. You should tighten hand tight
and then another turn or two with a wrench. When you're done,
half or more of the width of the original Teflon tape is usually
hidden inside the connection. In the photo, assuming normal
half-inch wide tape, it looks like almost the entire width of the
tape is still outside the joint.

It's already screwed in quite tight, maybe 3/4 to not-quite-one full
turn after finger tightening. I was afraid the brass might crack if I
tightened it more. Is that a valid concern? Or can brass stand that
kind of stress?

You *can* break brass parts if you try hard enough, so don't use a pipe
wrench with an "extension" on it.

However, it looks like very little of the Teflon tape made it inside the
joint. Did you apply the tape so it went all the way out to the end
thread on the elbow, or did you keep it back a ways from the end?
Teflon that you can still see when the joint is screwed together is not
doing you any good.

Another thought: look at the threads on the elbow and inside the
solenoid valve. They should be smooth and clean, cut with a tap or die,
with no obvious discontinuities (except at the thread start and end).
I *have* seen Chinese-made compressed air parts that had die cast
threads, and the mold separation lines still present in the threads
prevented them from ever sealing properly.

Dave