Plumbing question. I'm not Mr Handyman, but I'm not helpless, either,
so please bear with me.

I have a very old house. All the plumbing is 1/2" galvanized lead
pipe. I'm replacing a sink in the garage. The valves under the sink
are brass, outdoor type. The lead pipe comes out of the wall, then the
brass valve, then another short piece of lead pipe, then the flex hose
to the sink faucet. I've done this type of thing many times before,
but this time the pipe coming out of the valve just will not seal.
I've replaced the valve itself. I've tried different pieces of short
pipe. I've cleaned the threads. It just will not seal. I'm using
gray pipe thread compound with the brush, but it's a few years old and
has separated. I can stir it together, but I'm wondering if that may
be why it's not sealing. The cold water side and the rest of the hot
water side has sealed fine.

Should I go for an indoor-type valve? Should I get new thread
compound? Is there a better type of compound or sealer that I should
try? Any thoughts would be most appreciated. Thanks.