I made a quick fix washer out of a matchbook cover, on the same pressure
relief valve while on the road this summer.  The plastic valve was not
damaged but the side of the washer had blown out.  But you're right.  You
open the seat to look at the heater and there it sits peeing like a baby!
Raced to the nearest town and got a washer that fit at a auto parts store.

I remember that the washer had to have a certain thickness or it did not
cover and seal the relief groove in the valve.

Cost maybe 15 cents.

Alan :)

--
Alan Torrigino
McCloud High School Mathematics
McCloud, CA