I must apologise to Garfieldghost and others here. I just finished reading some very interesting WABO tech documents and reading some old forums on vacuum systems fitted to export US locomotives
It would seem and I can't confirm it. That the standard method that US locos used was air priority . the driver operated the train brake , which was air operated on the loco and behind the scenes the pneumatic automation combined with valves ensured that the vacumn was slaved to the air brake.
The driver merely treated the train brake as if it was air , and the control system reduced the vacumn in proportion to the loco air pressure
In that respect both the air and vacumn were " effectively " self lapping , ie the handle could be left in any position from barely on to full service and the system automation maintained a constant vacumn by relay valves connected to the exhauster. All this was transparent to the operator. Since both the air operated loco brake and the vacumn train brake were capable of progressive or partial release , the lever coould be moved back and forth in the service " area " and the system adjusted both the loco air brake and vacumn behind the scenes as appropriate. This pneumatic automation operation , in effect created a self -lapping vacumn brake. ( actual self lapping vacumn was quite rare as I said before )
The primary reason for this is US locos were typically built with train capable air compressors and an assumption that eventually operators would dump vacumn and switch to air, in fact the arrangement mentioned above allowed for a very simple switch over with no additional work by the driver
It also meant for multiple unit working , both the loco air and vacumn connections had to made between locos , I presume that was the case with the CIE EMDs.
It's turns out the whole system was more complex then I thought. In fact there may not have been a vac gauge in the cab ( was there ? )
Note that still isn't EQ brakes which were quite differebt
Now making an equivalent Dcc brake is virtually impossible, because variable application of brake force isn't possible , there is just one service level brake deceleration ( in Zimo this is cv 349 ) so the best that can be simulated is the effect of moving the train brake lever to one fixed position, holding the brake on ( lap ) and then full release . Which would mean the Dcc throttle , brake lever would have two positions , on ( and lap ) , off with release
But mike84c , the Dcc throttle is really cool isn't it. I have the bits coming to build one up but there's a lot of bits to make it all work. The faceplate is too big for my little cnc mf50 proxxon mill so that will have to made elsewhere ( Iowa scale engineering have only produced prototypes to date )
Ps. I also worked out what Release Auto was meant to be on the 141 brake console legends , it means that the loco brakes are released while the train brakes remain on !! , its spring loaded position and now the video on the DCDR cab ride in a 141 finally makes sense. Woohoo
Dave