a yoke 15 take up pin 23 jam sleeve or collar 28 and nut 29 and is adjustably secured on the yoke 15 in slots 20 in a manner similar to the first disclosed form of the brake unit as in the first form of the brake unit the front brake unit 12 is substantially identical with the rear brake unit and accordingly only the rear brake unit 13 will be described an adjustment pin 27 39 is adjustably clamped in bore 25 of take up pin 23 by the jam sleeve 28 and an elongate support bar 48 is suitably secured at one end thereof to the other end of adjustment pin 27 39 and extends substantially perpendicular thereto a pair of parallel support arms 49 and 50 are pivotally connected at one of their ends to the bar 48 adjacent opposite ends of the bar by means of pivot pins 51 and 52 respectively an elongate substantially rectangular brake shoe 30 39 has a pair of upstanding lugs 53 and 54 attached to the fins thereof adjacent opposite ends thereof and the other ends of arms 49 and 50 are pivotally connected thereto by means of pivot pins 55 and 56 the brake shoe 30.