A few days ago I noticed about a capful of water in the pail at the bottom of my water heater's TPR valve outlet. This is the first time I've had any water come out of the TPR when I wasn't testing it.
I moved into this house in 2011. In 2012 I installed a water expansion tank, with a ball valve to disconnect the hot water system—the expansion tank plus water heater. I realize the water expansion tank is probably near the end of its life. The water heater is electric and over 20 years old (I know), but I've been flushing it thoroughly once a year and replacing sacrificial anodes as needed, and it hasn't leaked yet.
The only recent change to the system is that I used to have a pressure gauge connected to a hot water faucet instead of the normal connection to my washing machine, and I finally removed the gauge and reconnected the hot water hose for the washing machine. When the washing machine runs, I can hear a knock in the pipes when the washer hot water valve turns on or off (which was why I installed an expansion tank in the first place). Note that there is a PRV on the water main.
To test things, I opened the TRP valve and closed it against. It took its good old time closing and kept leaking for a while. So I opened and closed it again, and that time it closed more quickly and stopped leaking. I had seen this once before in testing, so I didn't read anything into it.
Then I closed the ball valve to remove external pressure from the hot water system, I opened all of the hot water faucets above it, and I checked the air pressure inside my expansion tank. It's down around 28 PSI from the 60 PSI that it's supposed to be at, so I'll have to address that. But when I opened the ball valve again and individually closed each faucet, the TPR valve spit out a bit of water again when I closed the last valve.
Am I correct in my assumption that the TPR valve is failing? Is it normal for them to fail gradually instead of opening catastrophically? Would the underpressured expansion tank have any bearing on this?