An internal pump backpack sprayer system includes a first tank, a second tank and a pump unit. The first tank includes a tank housing for holding a fluid. The second tank is received within the first tank and receives a pressurized fluid. The pump unit has a cylinder housing with first and second inlets and first and second outlets, a piston within a cylinder and a pressure discharge assembly. When the piston moves in an up-stroke, fluid is drawn from the first tank through the first inlet while all or some pressurized fluid is discharged through the pressure discharge assembly with a remainder being discharged to the second tank through the second outlet. When the piston moves in a down-stroke, fluid is drawn from the first tank through the second inlet while pressurized fluid is discharged to the second tank through the first outlet.