apparently there's an incomplete load-screen tip in the game files about a compression brake, so they might be adding one in the next update.
I'd love air brakes! Some models have air brake gauges and I have been waiting patently for the day that they would be implemented! It is so satisfying mowing down traffic with the T series plow, and I have been curious how air brakes could contribute to the simulation. This idea has been suggested since 2015. I hope there is some debug UI elements that show the pressures for different parts of the plumbing, similar to how the power train graph is. I'd think that it would add some unique challenge to the bus route feature too. It also stands out like a sore thumb that you hear that "FTSHH" when using the T series brakes.
I thought I saw somewhere where a Jake Brake for the T-Series was not planned to be implemented. I really hope the developers change their mind because a Jake Brake would be really helpful for going down hills with heavy loads. IE a Jake Brake that is customizable with vehicle configuration (CTRL+W), and that can be activated like in real life would be really cool!
