My garden faucets each have a valve in the pipe going to them which has
a built-in bleeder valve, to drain the water in the pipe after the water
to the pipe is turned off.

The bleeder valve is opened by unscrewing a cap nut. The cap nut on one
is damaged -- most of the knurling is gone -- and it's impossible to
turn without pliers and I think eventually I will crush the nut.

Can I put on a replacement if I do? IIRC I'm almost certain there was
a stopping point that will make it hard to take the nut all the way off
(a convenience when I'm up on a ladder and there's lots of junk below me
to lose the nut in, but maybe a big inconvience if I need to remove
replace the nut) and would make it hard to put another back on?

Without a cap nut, I won't be able to turn on the water to the faucet
outside, because it will drip or run constantly. Surely they thought
of this and sell replacement cap nuts but I don't know what to search
for. Googling, I'm not even sure bleeder is the right word.

Standard copper pipe for a 1980 house garden faucet is 1/2", right?