Brake vans - why?

[PupCuff]

Having never worked with vacuum stock, am I correct in thinking the vacuum holds the brakes off (assuming springs apply them?) - the fail safe being loss of vacuum applies the brakes? Or is this completely incorrect?

Each bogie has its own vacuum brake equipment - a cylinder and a reservoir. The cylinder will remain constant with the pressure in the brake pipe, while the reservoir will (roughly) remain at the lowest pressure attained by the system - usually a vacuum of 21 inHg - due to a one way valve preventing air getting in (the odd leak is to be expected here and there but nothing significant).

So if you lose vacuum in the brake pipe, the air pressure in the cylinder (which is at the same pressure as the brake pipe) will be higher than the air pressure in the reservoir which will apply the brakes due to the pressure differential between the two sides of the system.

After a while, the odd leak in the reservoir will result in the pressure differential reducing and the brake force reducing, and eventually if left long enough both sides of the system will be at atmospheric pressure and the brakes will be fully released.