How long does a proportioning valve last?

For example, I had a leaking clutch slave in my car for nearly 10 years until the seals finally gave way.

My prop valve started leaking and I replaced it with one from the junkyard. If this one starts leaking, should I just leave it be?

I just can't afford the $200 for dealer one, and I have no way to know when these junkyard parts are going to go, and really, what's going to happen if I just let it slowly leak.