There probably is a fix, but it may be more in time and money than it's worth. If it was mine, I'd replace it. I looks to be fairly old and if repaired, it would likely need a complete rebuild. Just set the new one to the desired pressure and match the expansion tank to that pressure. Yeah, you need to buy a pressure gauge if you don't have one. The expansion tank pressure is checked with a tire pressure gauge and air added with a tire pump.