If you don't get the air out of the system, when it gets heated, it will expand much more than the expansion tank can handle, and you might have damaged the new one you put in. They are not designed to have the bladder fully extended. Air expands and contracts much more than water does, so having a little air in the system can cause pressures to vary much more than just having water in there.
Do you have an automatic air extraction system on the boiler lines? If not, I think you should have one added. But, you also have a leak somewhere. It may go away when you can limit the pressure to the normal range, which is often between one atmosphere (around 15 pounds) and 20 or so. The T&P valves often pop at not much over that pressure.