I have a Kubota L2501, I’m have problems with the hydrostic transmission. When you operate the tractor forward the peddel goes all the way down ( like it goes down on its own) and is very jerky, reverse is the same way but not as bad. When it is moveing forward and start up an incline the power goes away. I have changed the fluid and spin on filter does mine have a suction screen also that could be plugged. All the other hydrolics work fine.
There are some known problems with this transmission. First check that the pin is not broken at the HST pedal and the pedal is actually engaging in both directions and moving the shaft. If OK then check your filter/strainer for metal/debris that would indicate an internal problem or charge pump problem. Also it is recommended to have your charge pressure and transmission pressures checked and verified to OEM specs. This will confirm a pump or internal problem in need of transmission repair/replacement.