I have an 11 year old pressur tank. I use it seasonally for irrigation. When I drain the system prior to winter I shut off the power, open all of the hose bibs and once the water stops flowing I open a gate valve located 9 ft. from the well house and 18 in. below the surface. The remaining water in the system then flows rapidly into the soil. Inside of the well house I have another gate valve to drain any residual.and there is usualliy very little left, maybe a pint at most. A few days ago I followed the same drill only this time there was several galons left in the pressure tank and I kept opening and closing the inside gate valve to drain the pressure tank about 1 quart at a time. I heard "burping" in the tank as air was sucked back in to allow further draining. I've never heard the "burping" or encountered this draining problem before and want to know if my pressure tank needs replacing or what else might be going on.

want to know if my pressure tank needs replacing
No, presumably the piping from the well is piped into the bottom of the tank and the pipe to the house is teed of the well pipe. Just takes time for air to be sucked in so the water can drain out. Would drain faster if you had a drain valve directly on the tank. I have a pressure tank that is over 30 years old, yours is just getting broken in.

PS
I am taking you at your word that you have a pressure tank, not a bladder tank.

That would be my questions. Would you please clarify the type tank that you have. Is it a galvanized pressure tank or is it a bladder tank? Back to you, Tom