40 of t shaped transverse cross section and having an enlarged pressure pad 42 adjacent the outer or free end at the opposite end the lever 40 is pivotally secured by a pin 44 to a pair of upstanding brackets 46 disposed on opposite sides of the web portion 48 of the lever in sandwiching relation the manually operable lever also includes a tail portion 50 extending downwardly between the spaced apart lever support brackets 46 the tail portion has an arcuate lower end formed with a plurality of ratchet teeth 52 adapted to cooperate with a single upwardly projecting tooth 54 formed on the upper edge of a pawl member 56 intermediate its ends as shown in fig7 the inner end of the pawl is pivoted on a cross pin 58 extending between the brackets 46 and parallel of the lever pin 44 a u shaped leaf spring 60 is mounted between the lower edge of the pawl and the upper surface of the floor base 24 and biases the lever in a counterclockwise direction as shown in fig6 so that the pawl tooth 54 normally engages the ratchet teeth 52 on the lever tail portion 50 engagement of the pawl tooth 54 and the ratchet teeth 52 positively prevents clockwise rotation of the lever 40 about the pivot axle 44 and in order to release the lever the outer.