On full scale railways, the vacuum in the train pipe effectively held the brakes off. By opening that pipe to atmosphere, the vacuum, acting on the other side of the diaphragm applied the brake, because the pressures (vacuum) were no longer equal. (GWR ran with 25" Hg, everyone else used 21" Hg. So coupling a non GWR loco onto stock that had just been hauled in by a GWR loco required the reservoirs to be emptied, by "pulling the strings" so that the new loco could make a brake with equal vacuum on both sides of the diaphragm in the brake chambers )

On a model loco and train, the train pipe is more likely to be simpler, applying a vacuum to one side of the chamber, to either hold the brake off, so that is applied by a spring, or to apply the brake, as commercial vehicles once used to do.

HTH

Howard

Yes, it has to be!

On passenger trains, as well as the control valve in the c ab of the loco, in the guard's van there was a "setter" which if lifted, opened the train pipe to atmosphere, so destroying the balance in vacuum between the two sides of the atmosphere, so that the brake was applied.

If you look at pictures of preserved locos, (most used the vacuum brake, only a few used the Westinghouse compressed air brake ) you will see that on the front buffer beam, the swan neck flexible vacuum pipe is fixed to a blanking fitting.

The far end of the train pipe is sealed by the setter. The vacuum is supplied by a steam powered ejector, or a cross head driven pump on the GWR.

But that is full scale practice. On model locos, the decision has to be between using the vacuum to hold the brake off and apply it with a spring when the vacuum is destroyed. This is much like the spring brake system now used on commercial vehicles. If the compressed air supply fails, the spring applies the brake.

On older vehicles fitted with the triple vacuum servo servo system, the front brakes were applied directly by the vacuum being admitted to the brake chambers, while the rear brakes were applied by the driver's pressure on the brake pedal, aided by a vacuum servo cylinder, operating on rods to the rear brakes.

HTH

Howard

No problem, happy to help explain, if i can.

Which system is used will depend on the choice made by the owner of the model rolling stock.

The spring applied brake is fail safe, in that if the controlling medium is no longer available, the brake is applied.

Howard