If it leaks with the heat turned off, then it is a bad valve. The odds of
fixing it by opening it repeatedly is very small.
You can replace the valve cheaply enough, but then you still have a 12 year
old heater with a leak. Get rid of it now while it is a small leak rather
then in 2 months when it is a major leak, and save the $10 the valve will
That it leaks more with the heater on is probably not a problem; my pressure
can temporarily go from 50psi to 80psi when the heater cycles (some day I
will get around to installing an expansion tank), but it could be a problem
if it is going too high. For $5 get a pressure gauge and check it.