After you change the hydraulic fluid, idle the tractor for a good 10 minutes so the fluid circulates nice. Then you can run it. After the tractor has been parked, the fluid may go down a little. If the 1510 is like the 1710, the hydraulic dipstick is under the seat, behind the shifter. Make sure when you check the fluid, the dipstick is not screwed in, it just wants to sit on the threads.