I haven't seen grade assist use the friction brakes. Grade assist will spin ICE to very high rpm though to maintain speed. And if you shift to low, you immediately engage ICE and can slow down without friction brakes most times. Once you slow down below the grade assist speed, grade assist no longer is applicable. Also, when the HVB is full and regeneration is not possible, ICE will spin up for normal engine braking like any car when left in drive. Most times though you can shift to L and the additional engine braking is sufficient to slow down if one plans / knows the road ahead.
What I do when I need to slow down, I shift to low, disengage grade assist and when I've slowed enough engage grade assist and shift out of low and repeat as needed especially when entering a down hill hair pin turn. It works remarkably well.
I have monitored brake pressure when dropping IIRC about 1700 feet in about 4+ miles or so with several hairpin turns and many switchbacks. The friction brakes were never applied. For those that have traveled between Globe and Show Low AZ on US 60, it's the stretch of 60 dropping down and crossing the Salt River.
Edited by Plus 3 Golfer, 27 May 2016 - 09:25 AM.