My first thought is don't use it for a tractor. The brakes are probably screaming for mercy.
I hope you live someplace that is pancake flat. A downhill run could be a killer. Literally. Water weighs something over 8 pounds per gallon. So 20 gallons is something over 160 pounds, not counting the sprayer itself or the weight of the driver.
Depending on which model Murray you have I guess if the brake is external you can take it apart and see if the pads are still in there.