A three-position booster valve mechanism comprising: - a housing having an inlet port, an outlet port and an exhaust port; - a supply passage extending between the inlet port and the outlet port; - an exhaust passage extending between the outlet port and the exhaust port; - a supply valve provided in the supply passage and operable between a closed and an open position, in which the supply valve is biased by a spring towards the closed position in which it closes the supply passage; - an exhaust valve provided in the exhaust passage and operable between a closed and an open position, in which the exhaust valve is biased by a spring towards the closed position in which it closes the exhaust passage; - an operating member for moving the supply valve and the exhaust valve from their closed towards their open positions; - an operable piston-cylinder control system for moving the operating member between a first position in which the supply valve and the exhaust valve are closed, a second position in which the supply valve is open and the exhaust valve is closed, and a third position in which the supply valve is closed and the exhaust valve is open;and - a pilot port which connects to one side of the piston-cylinder control system for moving the operating member in between the first, second and third positions in dependence of a pilot pressure delivered to the pilot port, in which the piston-cylinder control system at its side opposite the side which connects to the pilot port, is provided with a first medium passage which connects to the outlet port, characterized in that, the piston-cylinder control system connects to the operating member at a central point of application in between the supply valve and the exhaust valve, the operating member extending in opposing directions from said central point of application towards the supply valve and the exhaust valve. - a housing having an inlet port, an outlet port and an exhaust port; - a supply passage extending between the inlet port and the outlet port; - an exhaust passage extending between the outlet port and the exhaust port; - a supply valve provided in the supply passage and operable between a closed and an open position, in which the supply valve is biased by a spring towards the closed position in which it closes the supply passage; - an exhaust valve provided in the exhaust passage and operable between a closed and an open position, in which the exhaust valve is biased by a spring towards the closed position in which it closes the exhaust passage; - an operating member for moving the supply valve and the exhaust valve from their closed towards their open positions; - an operable piston-cylinder control system for moving the operating member between a first position in which the supply valve and the exhaust valve are closed, a second position in which the supply valve is open and the exhaust valve is closed, and a third position in which the supply valve is closed and the exhaust valve is open;and - a pilot port which connects to one side of the piston-cylinder control system for moving the operating member in between the first, second and third positions in dependence of a pilot pressure delivered to the pilot port,