I used to try this coming into Antonito in the afternoon. I could only get it to work at speeds around 20-22 and only with 487. I remember doing it one afternoon with Jim Shawcroft firing for me.
He was sitting over there studying me, watching my every move. He finally got up wandered over to my side, studied where I had the brake valve handle on the "bridge", shook his head and said something about "damned punks" and went back and sat down. He later told me he could never get the hang of doing that, but a few old heads could. He said it worked better with long trains that took more air to hold back.
I don't think I'd try that coming off the 4% though.
I also heard about guys "feed valve braking" where you'd take a 10 lb set, turn the feed valve back a bit, then put the valve back in run. The feed valve would now be set at a lower pressure, and maintain that pressure.
BTW, Westside Lumber used WABCO's combined straight/automatic air system. Between the triple and the reservoir is a filler piece with a valve inside which is where the straight air line goes in. Unlike the D&S system which feeds the air in through the retainer line, with WABCO's system you can used both systems independently - including the retainer valves, although I belive, the automatic had to be released in order for the straight air to release. With the D&S system, the straight air will only work if the automatic is in release as this is the only position where the triple valve exhaust port (which is where the straight air goes in) is open to the brake cylinder.