A overrun brake system includes a gas spring with a gas-tight cylinder and a piston with holes allowing gas to slip through so that the pressure is the same on both sides. Since the free end of the piston has a larger area than the side where the piston rod is located, the gas spring will act as a compression spring. To prevent the overrun brakes from being activated during reversing, the gas spring has a solenoid controlled valve receiving current from the reverse light circuit and closing the flow of gas through the piston when the towing unit's gear is set into reverse. When the gear is set into forward position the solenoid loses its current and a mechanical return spring opens the valve, so that the overrun brakes may act normally again.