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And whilst we are talking about GWR vacuum brakes, don't forget that they used a higher vacuum pressure than other UK railways. When a GWR loco came off the train and was replaced by another loco the brakes on each vehicle had to be manually released.
In Open Rails air brakes have a function called the "bleed off valve" which is the equivalent of the "release valve" on vacuum brakes. If this can be made functional for vacuum brakes then it will be possible to represent this.
In the present version TrainBrakesControllerMaxSystemPressure ( 25 ) can be used to operate GWR vacuum brakes at 25 in Hg (rather than 20 or 21 in Hg).