Railworks America

RSC says it was fixed with the 10.9a update.

Well, I think it isn't. At least not with my brand new K4 I just got yesterday. It does seem to work on some UK locos, either vacuum braked or Westinghouse. It is dead on the BR52 however.
It doesn't work in Challenger, Big Boy or K4 nor in any of the GTrax steamers. How are supposed to descend grades without this important indication?

To which of the control values in the engine control container component is the HUD linked? I suppose to TrainBrakeCylinderPressure though names do very between engines. Perhaps this is the cause, otherwise it is a core issue still.

All I can advise is to not use that cartoon F4 HUD, use the F5 (hit the key twice) You will see all the info you need. Also, use the F3 Driver's guide. Oh, hit F4 once to turn it off. Then F5 twice.

Or, I dunno, read the cab gauge :p it does have one, right? the cab looked a bit bleak when I glanced at the screenshots.

The control for train brakes is called whatever you feel like, as long as it's the same in the engine & simulation blueprints. Generally it's TrainBrakeControl though.