Confusing puzzle to do with pressure?

Your father and your younger brother are confronted with the same puzzle. Your father's garden sprayer and your brothers water cannon both have tanks with a capacity of 5L. Your father puts a negiliible amount of concentrated fertiliser into his tank. They both pour in 4L of water and seal up their tanks, so the tanks also contain air at atmospheric pressure. Next, each uses a hand-operated piston pump to inject more air until the absolute pressure in the tank reaches 2.40atm and it becomes too difficult to move the pump handle. Now each uses his device to to spray out water, not air until the stream beacomes feeble as it does when the pressure in the tank reaches 1.20 atm. The he must pump it up, spray again and so on. To accomplish spraying out all the water, each find he must pump up the tanks three times. Here is the result of the second pumping. The !st and the 3rd pumping processes seem just as difficult as the 2nd, but result in result in a disappointingly small amount of water coming out. Account for this phenomenon