A coolant sensor doubles as a bleed valve for purging trapped air from a cooling system. The sensor slides into a port placing a sensor element in contact with the coolant. The sensor includes a pair of encircling o-rings for sealing the system from atmosphere in one position and permitting bleeding in a second position. The bleed channel is in the sensor port between the o-rings. The sensor is moved so the innermost o-ring clears the narrow opening into the cooling system. The outermost o-ring maintains a seal to keep the coolant from leaking out of the sensor port. The sensor is held in either position by a horseshoe clip or by a bayonet arrangement, the horseshoe clip arrangement including multiple apertures for each position, the bayonet having a position to provide for safe bleeding of air from the cooling system without completely removing the sensor from its retention port.