First turn off the motor, make certain your implement/3 pt hitch is lowered and the lever has been released.
Check the hydraulic oil volume.
The dipstick on our 1510D is on the operator's left and under the seat. Level should be "at the top of the flat part .. or slightly below". You might want to clean the area around the dipstick before you pull it out - dirt in the hydralic/transmission system clogs small openings & wears bearings.
If the fluid is low the filler is a flat knurled knob further back & close to the tractor's centerline. Use "JD 303" equivalent fluid.
Then check two things: the implement drop speed valve & the hydraulic oil screen.
The drop valve is under the seat front. Look for a lever that turns. Clockwise slows drop speed, your may have been turned counter-clockwise at some time. Also under the seat front should be the stop valve. Fully clockwise locks the 3-point system and stops it from moving.
The hydraulic oil screen on ours is behind a lobed plate, on the operator's left, toward the back and on the side of the transmission case. Remove the bolts & ease the cover out, may take some wiggling there's an "O" ring seal on it. Reach in & pull out the screen, it looks like a tube.
Clean the screen in freash gas or other clean solvent, dry & replace. If there was a gummy/waxy buildup, or anything else, on the screen you may have found a problem. Try draining out and replacing the hydralic fluid. Takes about 3 gallons, drain is under transmission case.
If none of these lessens or solves the problem you may have a leak somewhere. It's a bad idea to search for a hydraulic leak with the system under pressure. You can get the fluid in your eyes or, if the pressure's high enough, it can penatrate your skin - not a good idea.
A cursory look for leaks may find your problem. Check the fittings to/from the hydraulic pump. Tighten judiciously - you don't want to turn a simple drip into a $,$$$.$$ repair!
Good Luck!
cvt