Either you have worn out the brakes by running the tractor with the brake engaged, or you haven't applied enough pressure to the brake pedal. Once it starts moving, the brakes are harder to get the tractor to stop. The brake is only a brake to hold the tractor once it has stopped. It is not like a brake on a car that will stop the tractor once it gains speed. That is why the hydro is designed like it is. It acts like a brake when you take your foot off the control and it returns to neutral.
I have just checked the manual, and it refers to it specifically as a PARKING BRAKE and further says "If it is necessary to park on a incline, be sure to chock the wheels to prevent accidental rolling or the machine.". It is clear to me that Kubota never meant for the parking brake to be used as a service brake. I suggest that you re read your Operators Manual and familiarize yourself with the proper operation of the tractor before you get hurt.