> The old No. 6 and No. 8 brake valves did not allow rapid recharging of the air reservoirs on the cars,

And just why not?

>The long standing rule has been that the maximum brake pipe leakage cannot exceed 5 psi per minute.

Rule?? It's a law, a statute. While I'm not aware of the date it became a law, I do know that the principle has been around for at least 100 years because it's in the SP's red electric rulebook of 1912.

>26L, 30 systems and the WABCO/NYAB/KNORR electronic brake valve schedules all now have pressure maintaining, as well as the rare 6BL retrofitted with a NYAB system.

Last time I checked, the 26-L system used either a 26-L or 30-CDW brake valves. The "26" in 26-L refers to the control valve (a 26-F control valve), not the engineer's brake valve. There never was a time 26-L didn't have pressure maintaining because it's inherent in the design of the system.

>The use of pressure maintaining (eliminating the need to cycle brake in most instances)

It is to be pointed out here that "cycle braking", in its modern incarnation, is still needed for wheel cooling.

>The FRA in the last few years required the railroads to change their air brake rules. Too many railroads became reliant on dynamic brakes, and were operating trains incapable of control by air alone. Too many trains were having dynamic brake failures and engineers did not recognize the danger, and in trying to control their trains with air lost control.

If your referring to the changes of about 10 years ago which require that dynamics must be functional upon leaving the initial mechanical facility, these were brought about because the RRs were (and still are) coercing their engineers to use dynamics as the prime braking system on the train.