Water Heaters - Testing Expansion Tank on Water Heater

I had a plumber come to bid on replumbing the washer and laundry sink (due to remodelling, no problems with existing). He saw the expansion tank on my water heater and tested it by inserting needlenose pliers in the top. He said water should have come out. He then hooked a bicycle pump with gauge up to it.. the gauge needle didn't move. He said the expansion tank was not working because there should be pressure. The gauge he put on my laundry tub faucet read 50 psi. Is the plumber correct? The guys at work say that's not how they understood expansion tanks to work.

Thanks in advance

Sounds like you're expansion valve might be flat, but I doubt water should come out the air valve! There's a bladder between the air and the water. Get the operating manual for your unit off the internet and it will show you how to service it correctly with air. It could be bad, but didn't you say he pushed the valve open for awhile? Your pressure may have escaped then.

I believe older tanks might not have used a bladder. It would prob have to be pretty old though. The bladder type would never have water coming out of the valve, unless the bladder has failed.

Non bladder tanks, over time, the air can dissolve into the water rendering them ineffective. The solution is either replacement with a bladder type (recommended) or to partially drain and recharge the tank (I think), this would be a continuing maintenance item.

Does it have a brand and model number on it somewhere.

I'll check for a brand and model. It was installed 4 or 5 years ago with a new water heater. I don't mind having a new expansion tank installed when the other plumbing is updated. I just had no idea if what I was being told was even possibly true.
Thanks for the replies!