A liquid dispensing device (1; 101) comprising: - a piston chamber (4; 104) having a return opening (52; 152) in a side wall thereof; - a piston (5; 105) that is moveable within the piston chamber (4; 104) to pressurise a liquid to be dispensed, the piston (5; 105) comprising a first circumferential seal (39; 139) for sealing a part of the piston chamber (4; 104) between the piston (5; 105) and a piston chamber end wall (13; 113) and a second circumferential seal (38; 138) spaced apart from the first circumferential seal (39; 139), such that when the piston (5; 105) is at or near an end of its stroke, the return opening (52; 152) is located between the first and second circumferential seals; - a nozzle (6; 106) with a defined throughput for dispensing the liquid; - an outlet valve (7; 107) having a defined minimum opening pressure arranged between the piston chamber (4; 104) and the nozzle (6; 106); - a prime valve (40; 140) for priming the device, the prime valve (40; 140) being mechanically operable and being arranged on or in the piston (5; 105); and - an operating member (41; 141) in the piston chamber (4; 104) which is arranged to move the prime valve (40; 140) from a closed position to an open position when the moveable piston (5; 105) is near or at an end of its stroke, characterized in that the operating member (41; 141) protrudes from the end wall (13; 113) of the piston chamber (4; 104); and in that the prime valve (40; 140) comprises: a sealing part (44; 144) arranged radially inward of the first and second circumferential seals (38, 39; 138, 139) and closing off an orifice (50; 150) in the piston (5; 105); and an actuating part (46; 146) connected to the sealing part (44; 144) and arranged to cooperate with the operating member (41; 141). - a piston chamber (4; 104) having a return opening (52; 152) in a side wall thereof; - a piston (5; 105) that is moveable within the piston chamber (4; 104) to pressurise a liquid to be dispensed, the piston (5; 105) comprising a first circumferential seal (39; 139) for sealing a part of the piston chamber (4; 104) between the piston (5; 105) and a piston chamber end wall (13; 113) and a second circumferential seal (38; 138) spaced apart from the first circumferential seal (39; 139), such that when the piston (5; 105) is at or near an end of its stroke, the return opening (52; 152) is located between the first and second circumferential seals; - a nozzle (6; 106) with a defined throughput for dispensing the liquid; - an outlet valve (7; 107) having a defined minimum opening pressure arranged between the piston chamber (4; 104) and the nozzle (6; 106); - a prime valve (40; 140) for priming the device, the prime valve (40; 140) being mechanically operable and being arranged on or in the piston (5; 105); and - an operating member (41; 141) in the piston chamber (4; 104) which is arranged to move the prime valve (40; 140) from a closed position to an open position when the moveable piston (5; 105) is near or at an end of its stroke, a sealing part (44; 144) arranged radially inward of the first and second circumferential seals (38, 39; 138, 139) and closing off an orifice (50; 150) in the piston (5; 105); and an actuating part (46; 146) connected to the sealing part (44; 144) and arranged to cooperate with the operating member (41; 141).