As pointed out by others in some cases:

The equalizing reservoir and brake pipe pressure should match on the leading unit, though there is often a few pound difference in the gauges (which drives me crazy).

The FRA a few years ago declared that the standard brake pipe pressure for freight trains was 90 psi, so if the brakes are released that is what the equalizing reservoir will indicate. Mainline passenger trains typically use 110 psi.

To make a normal brake pipe application, the engineer manipulates the automatic brake handle to reduce the equalizing reservoir pressure the desired amount. The equalizing piston (or modern equivalent) "automatically" reduces the brake pipe pressure to match the equalizing reservoir pressure, and gradually closes of the brake pipe exhaust to prevent a pressure "bounce" that might trigger an undesired release. On long trains, depending on the type of control valves on the cars and brake pipe leakage, it may take quite a few seconds to exhaust the brake pipe to the desired level.

Older independent brake valves (for the engine brakes) are not self-lapping, with most engineers cycling between application and release in an attempt to obtain the desired braking effort. In between application and release is the "LAP" position, which simply holds the pressure. The newer brake schedules introduced a self-lapping independent brake valve = moving the handle to the left decreases the brake cylinder pressure, to the right increases it. You are actually controlling a regulating valve, so the resulting pressure is generally in proportion to the handle position.

The 24RL brake schedule appears to have evolved from the 8EL/8ET design. There are two generations of common instruction pamphlets for them. The first series was the D-24 and even included an ET version for steam locomotives. This was NOT a pressure maintaining valve.

Both the 8 and 24 systems have a pressure maintaining valve that functioned in the first service position. To make an initial brake application, the engineer moved the automatic brake valve handle from RUNNING to FIRST SERVICE, which caused a 5-7 psi drop in the equalizing reservoir and then continued the reduction at a very slow rate. The theory was that on longer trains initial quick service and brake pipe leakage could cause an undesirable high rate of brake pipe reduction and resulting slack issues - in such cases the pressure maintaining valve would add enough air back into the brake pipe to maintain the desired rate of reduction. Once the initial brake pipe reduction was made, the automatic brake valve was placed in the LAP position, additional reduction made by using the APPLICATION position and returning to LAP as usual.

Somewhere along the way, it was decided that keeping pressure maintaining in effect at all times was desirable. The common "tricks: up until that time was to either "bridge" the valve simply guessing how much air to continue to add back to the brake pipe to replace leakage (bad idea as a little too much and the brakes release) or to use the feed valve to simply reduce the standard brake pipe pressure to the desired reduction (now prohibited by the FRA). WABCO changed the D-24 to the D-24-M to include pressure maintaining during any brake pipe reduction, and also designed a retrofit to the previous D-24 to convert it to pressure maintaining re-designating it as the D-24-MC. Both of these included a handle to cut the pressure maintaining out to make a brake pipe leakage test as pressure maintaining now functioned with the automatic brake valve handle in the LAP position.

Rather than buy the expensive MC conversion option, Southern Pacific (and presumably others) discovered that by simply blanking off the first service function, the former FIRST SERVICE position acted as pressure maintaining. To make a brake pipe reduction, the engineer moved the handle through the former FIRST SERVICE now MAINTAINING position, through LAP to Application and makes the minimum brake pipe reduction as on older systems. Once the desired equalizing reservoir pressure reduction is made, the brake handle is either returned to LAP (not pressure maintaining) or to the MAINTAINING position. It is believed there may have been a stabilizing hole drilled as part of the conversion. A disadvantage with the 24 pressure maintaining system is that if the air (being trapped) in the equalizing reservoir becomes sufficiently heated, expands and increases in pressure during long brake applications an undesired brake release can result. (26/30 generally doesn't have that problem as the equalizing reservoir is controlled by a regulating valve arrangement).

24RL is a nightmare to maintain as each portion in the stack making up the automatic brake valve weighs about the same as a boat anchor. Because of the differences between the various factory options and subsequent pressure maintaining versions (factory, factory conversion and railroad conversion) the last I knew most air brake shops will only do a complete system repair and return, if they are willing to do it at all.

26 and 30 both came out of the passenger/transit side of WABCO. The feed valve was replaced by a relay valve that both adds air and exhausts the brake pipe pressure attempting to match the equalizing reservoir pressure. I guess you would also consider these as self-lapping brake valves as advancing the position of the brake handle through the application zone creates the corresponding equalizing reservoir and brake pipe reduction. But - there is an "E" version of both the 26 and 30 engineer's brake valve which is a position valve (not self lapping - has holding/recharge, lap and application positions) used with the electro-pneumatic PS-68 brake systems.

Not sure if I got all the designations 100% right without digging through manuals for hours. But as Dr. Z pointed out, WABCO/NYAB often tried early on to use ideas and materials that are commonplace today but in their time were not practical.