shaped nipples 37 and 38 to which are connected couplings 39 and 40 respectively to flexible hoses 41 and 42 the opposite ends of which are coupled to existing tire valves 43 and 44 in the respective inboard tire 15 and outboard tire 16 fixed to the piston body member 21 adjacent the piston head 23 is a first movable switch element specifically a magnet 45 a similar or second switch element or magnet 46 is fixed to the body member 21 adjacent the opposite piston head 24 and in longitudinal or axial alignment with the first switch element 45 the switch elements 45 and 46 move with the reciprocable member 20 fixed to the exterior surface of the pneumatic cylinder 19 are a plurality of three as shown in the drawings opposing switch elements 48 49 and 50 more specifically solid state proximity switches each being adapted to be actuated by the proximity of a movable switch element or magnet 45 or 46 as the switch element 45 or 46 moves axially until it is in substantially radial alignment with a corresponding switch element 48 49 or 50 in a preferred from of the invention the switch elements 48 49 and 50 are 34 micro switch 34 proximity hall effect switches the proximity switches 48 49 and 50 are held in their fixed positions upon the cylinder 19 by the clamp blocks 52 fig5 8 the detector device 18 is fixed to the central hub 63 of the outboard wheel 14 in such a manner that the longitudinal axis of the cylinder 19 is disposed diametrically of the hub 63 or the outboard wheel 14 the detector device 18.