> 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).

The #24 brake valve had several configurations: D-xxx-24-yyy, where
xxx =
A, safety control (deadman)
S, train control/locomotive overspeed control
E, electro-pneumatic, self-lapping, internal quadrant

and yyy =
H, hinged handle, internal quadrant
M, brake pipe pressure maintaining
MC, converted to pressure maintaining, filling piece change (the WABCo mod)
T, permanently attached handle for steam locomotives

Thus, a DS-24-M was a D-24 brake valve with train control/overspeed which had been modified for pressure maintaining using the plugged port method. A DS-24-MC was the same valve, except that it used the WABCo add-on/retrofit kit.

WABCo's add-on/retrofit used the Lap position for pressure maintaining, and there was a separate cut-out handle which had to be flipped to disable it for initial terminal leakage tests.

The "SP modification" used what had been the First Service position for pressure maintaining, but for leakage tests, Lap had to be used. IOW when slowing a train, make a service application, then move the handle to what had been First Service; to make a leakage test, apply the brakes and move the handle to Lap.

For pressure maintaining to work, WABCo specifies that the EQ res - EQ piston circuit be "bottle tight", which means "no leakage at all". In other words, once an application has been made, pressure in this circuit can't change at all because it's the reference for regulating brake pipe pressure to the desired amount of application.

> most air brake shops will only do a complete system repair and return, if they are willing to do it at all.

Correct. A friend who used to work for WABCo once told me that to perform a complete code of tests with test racks for a 24-RL system took one person 8 hours. Repairs these days are also a question replacement part availability, and the most difficult part to repair is the application piston and valve assembly.

> 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.

If by "30" you're referring to the 30-CDW type of brake valves, it's still the 26 system, the system designation being based on the control valve, which is still a version of the #26 control valve. Similarly, the #6 system (6-ET) is still produced, using a 26-C brake valve and a 6-N distributing valve.

The 26-E brake valves is manual lapping because of operating the internal switches used to control the magnet valves on the cars in the train and the desire to have a Holding position. It's possible to have a self-lapping electro-pneumatic brake valve, like a DSE-24 or MS-40-A, but these require more components along with their additional costs of purchase and maintenance.