I worked in two air shops. Rebuilding the valves isn't too bad as long as the slide valves are good. At the time, all the ABD/ABDW slide valves were always sent across the Lapmaster.

I maintain one loco with 14EL. This isn't too bad because the system is simple enough. The biggest issue is that the parts are just getting too worn, and no one is refurbishing the rings and bores. 6BL/BLC is similar and has the advantage of a self lapping independent.

There are several issues with 24RL. First, I am convinced it was the result of an all night drunk fest at Wabco. (Actually not - it is an update of the 8ET/EL - maybe it was the drunk fest and 24RL was the hangover?) There are so many variations of it that one contract air shop will not ship you a replacement set, but want you to send yours in for R&R. It is heavy, nasty and with ring and piston is worn out. It is a decent brake to run.

26L is my favorite to run and repair - but you have to understand it. Too many people can't get the relay valves right, and don't understand that the combination of valves and piping determines the variations in operation of 26L/26NL. Conversions aren't cheap - it usually runs over $10k just in parts, plus trying to cram all the piping and reservoirs in the available space. 30 has even more variations and Wabco doesn't want any independent shop to have the test plates. I hate running desktop controls.

Biggest problem with air brakes nowadays - hardly any engineer really understands the basic operating principles or physics behind it, and even fewer con repair or modify it.