OldBUFFkeeper
BTW, am less than impressed with the "air bleed" version of the J58, as the J57-p43WB, of which the B-52G I crewed, nearly a half century ago, had eight, featured the same thing, and for the same reason, i.e. to keep the N1 low pressure compressor (think first four stages of the single-spool J-58) from stalling when the engine was throttled back. except instead of dumping into the AB, (the BUFF didn't have any!), it simply vented straight out laterally, and would knock you on your can if you happened to be under it when it opened, as I was a couple of times.
The bleed valve used on the J58's used on the A-12/YF-12/SR-71A/M-21 were not designed for exactly the same purpose. On the J57 as you describe, they were to deal with the fact that the LP compressor was drawing in more air than the HP stages could handle, so they put a bleed-valve in to get rid of the excess air until the RPM increased passed a certain point. Earlier engines also used bleed valves too, with single spool engines it was actually worse as they only had one shaft spinning at the same speed which made the problem worse and required either proportionately larger bleed-valves, and/or more bleed-valves to work as well. A twin-spool engine at least has the luxury of the LP spool spinning slower.