Depending on the situation, there may be no time for downshifting. If I'm approaching an intersection or other cars, and I lose my normal hydraulic brakes, my reaction will probably be to yank it to nuetral and stomp hard on the "parking" brake. I'd rest a little easier if my "parking" brake was giving me more grab. I'm mainly interested to know how other trucks behave under these conditions. Of course the thought of dual master cylinders and disk brakes has crossed my mind, but for now I'm staying original. Thanks.
