The pressure overflow valve on my furnace is leaking. It's a gas
fired hot water radiator system. I recently replaced the corroded
original, thinking that that was the problem, but it is persisting.
Here are the symptoms: The pressure gauge on the unit shows 30psi
which is the burst point for the pressure valve, hence the leak. If I
trigger the valve and fill up a bucket with water, I can get the
pressure gauge down to around zero, but within an hour it's back up to
30psi. I think the problem may be that the water supply valve to the
furnace has failed, allowing water to get by and raise the pressure.
Does that sound right? It feels like there's water flowing in the
supply line after I trigger the pressure relief valve, but the supply
has a ball valve, and I thought those things never failed...Could
there be some grit or corrosion in it? I want to make sure I
eliminate other possibilities before I cut the valve out and replace
it.

Thanks, Andrew