The A-9 valve is a self-lapping, pressure-maintaining automatic brake valve used to control the brakes of the entire train (formation + locomotive) — i.e. both “train brakes” and “loco + train” in combined mode.
Purpose of A9
- Apply brakes to the whole train via the BP.
- Graduated brake application → smooth stopping of train.
- Emergency brake application → rapid BP drop.
- Works in combination with SA9 (loco brakes) for full control.
A9 Brake Valve – Key Functions
| Function | Description |
| Controls Train Brakes | A9 regulates Brake Pipe pressure (BP) to apply or release brakes on the entire train. |
| Charges the Brake Pipe | In Release / Running position, A9 supplies air from Main Reservoir to BP to release brakes. |
| Graduated Brake Application | Small handle movement reduces BP in controlled steps → proportional brake application. |
| Maintains Set BP Pressure (Lap) | In Lap position, A9 holds BP at the level set by the driver by sealing both supply and exhaust ports. |
| Service Braking | Provides smooth braking by gradual BP reduction, avoiding jerks for passengers/rolling stock. |
| Emergency Braking | In Emergency position, A9 dumps BP instantly to atmosphere → full brake cylinder pressure → rapid stop. |
| Self-Lapping Mechanism | Automatically balances air supply/exhaust to maintain the selected pressure without constant driver input. |
| Quick Release Capability | Quick BP recharge after brake application for fast break release during acceleration or traction. |
| Fail-Safe Safety Action | Any sudden BP drop = automatic brake application (inherent fail-safe property of A9 brake system). |