Just a suggestion- - - -I'll bet one of our talented tinkerers could adapt a hydraulic rear disc brake from a 3/4 ton or 1 ton truck to the output flange of an SM-420, and use a small master cylinder operated by a ratcheting hand lever to apply it. Since OEM parts are either missing or broken on a lot of trucks, a tech tip on how to fabricate one might be a good idea.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!