u shaped elbow bracket 42 at pivots comprised of opposed pairs of screws 44 and 46 respectively to thus form a parallelogram linkage of the links and brackets the base bracket 36 receives a base pin 48 having a lower end received within the socket 20 the upper end in turn received in a socket 50 formed into the lower face of the base bracket 36 and secured with set screws 52 the opposed pairs of machine screws 38 40 44 and 46 pass through aligned pairs of bores formed through sidewalls of the respective base bracket 36 and elbow bracket 42 and are threadedly received into bores formed in the sidewalls of the u channel links 32 and 34 the machine screws 38 also provide the friction brake adjustment function as will be described hereinafter the parallelogram linkage of which the rear arm 30 is comprised creates relative linear movement between the u channel links 32 and 34 as these elements rotate together about the various pivot points during swinging motion of the rear arm 30 such linear motion is resisted by a pair of tension springs 54 and 56 tension spring 54 is anchored at one end of a weld pin 58 secured to the u channel link 34 and at its other end anchored by means of a tension adjusting arrangement 60 carried by the other u channel link 32 while tension spring 56 is anchored to pin 62 carried by the.