icefire has quit [Read error: Connection reset by peer]
<Pinkbeast>
Possible, yes - air brakes have often been self-lapping, at least since the 50s, maybe before.
<Pinkbeast>
AFAIK it was never done on vacuum brakes. I suppose it could have been but of course the initial compelling advantage of the vacuum brake - well, the initial advantage was that it had been invented and the air brake hadn't, but after that the advantage of the vacuum brake was it's a much simpler device with fewer moving parts
<Pinkbeast>
Also often the small ejector would be left on & since the rate of loss of vacuum at a given brake handle position is proportional to the pressure differential, but the ejectors suck air out at a constant-ish rate, there's effectively a steady state with the brake valve slightly open.
<Iskierka>
You can lock it at a steady state, but you can't, for a really long train for instance, set a pressure you want it to get to, then leave it at that while you fine-tune speed into station using the loco power
<Pinkbeast>
... if, and now I'm reading up new things, the brake valve doesn't automagically disconnect the small ejector from the train pipe anyway, which some do.
<Pinkbeast>
Iskierka: That's correct. You'd never want to run with brake on and regulator open anyway
<Pinkbeast>
You get to your target pressure by opening the brake valve, then close it, then fine tune by opening it a bit or by using the large ejector (which may be controlled by moving the brake handle the other way from the "closed" position, or a separate control)
<Pinkbeast>
opening it _for_ a bit, I mean
<Pinkbeast>
Oh, no, here is a picture of a DMU vacuum brake valve with a lap position
<Pinkbeast>
No, that's not the distinction they're drawing - neither is self-lapping