A self-lapping brake valve laps itself to a certain pressure based on the position where you have moved the handle through its "application zone". With a manually lapped brake valve, you have to move the handle to apply and then to lap. This applies to straight air, independent, and automatic brake valves. Oddly enough WABCo's A-11 brake valve of 1876, the first one specifically designed for automatic air, was self-lapping, but its pressure regulation abilities weren't all that good.

> if the brake valve had been through the shops, there was a brass tag with the letter "M" riveted to the valve. That showed it had been converted to pressure maintaining in the old lap position.

The former first service position, and the valve becamse known as a DS-24-M, the M standing for "maintaining". This was accomplished by plugging a port in the rotary valve seat. WABCo made a somewhat pricey pressure maintaining add-on which didn't do away with first service. In the mid-1970s, a lot of the SP's SD-7s had this version (numbered in the 2700 series, until they were rebuilt). In order to keep things perfectly clear, this modification also added that M suffix to the brake valve ID.