Oil has a high expansion factor with heat. So if the cylinder is warmer than when it was unhooked then the balls will have pressure against them.
Solutions to the problem:
1) Loosen a fitting and let the pressure off.
2) Update the female couplers on your tractor to ones that enable you to hook hoses under pressure.
You did not tell us what brand or model of tractor you where using. Then we will know what hose ends you are dealing with. The Pioneer/ISO tips just became a universal standard in the early 1990s. Ford ran their own style of ISO fitting, had square end rather than the ball tye. IH had their own completely different fitting. JD had their own too.
DO NOT beat the hose tip with a hammer. That ball you are seeing is a check ball and has to seal when closed. Damaging it will cause it to leak when not in used. You can push it against some thing steel and not hurt it but don't "beat" on it.