with transverse legs 38 of t slots 32 34 at least one of the drive wheels 14 16 and preferably both is provided with retaining mechanism broadly identified by the numeral 70 detailed in fig6 7 and 8 for selectively releasably holding the drive wheel against rotation each retaining mechanism 70 may take a variety of different forms including for example a lockable park mechanism including interengageable mechanical locking components or a parking brake in the illustrated and preferred embodiment each retaining mechanism 70 takes the form of a parking brake 72 which is preferably a friction brake an actuator lever or member 74 projects through an upright slot 76 in a stationary rear wall 78 of brake 72 and is moveable up and down between an upper actuated position in which a braking force is applied to hold the drive wheel against rotation and a lower released position in which the braking force is released from the wheel to permit it to rotate in one preferred form of the invention each brake hydraulic motor and drive wheel hub may comprise an integrated unit such as that available from white hydraulics inc of hopkinsville ky as a model ce motor brake retaining mechanism 70 further includes a tension spring 80 operably coupled with brake member 74 in a manner to yieldably bias member 74 toward its raised actuated position thus applying the brake tension spring 80 is connected at its upper end to an adjustable eye bolt 82 that is secured to a mounting bracket 84 on the outside of an upright formed metal generally channel shaped tower 86 that is secured to and forms a part of chassis 12 the lower end of tension spring 80 is connected to the outer end of.