valves on bottom of TM

I have a 90 M23 that I bought used about 2 years ago. On a recent trip one of those valves on the bottom (2 in front, 2 in back that you drain the TM for winter ) started a slow drip. I wa s unable to stop it. Does anyone have any advice on repairing or replacing these.? :)

I've never replaced one, but I did notice that once when I had forgotton my water pressure regulator( the in line type),I could not get it to quit leaking while I was hooked up to city water.  Once I made a trip to the campground store and bought one there was no problem.  Thias may not be your situation at all, but thought I'd mention it.  Denny