Ooops, that was truncated. The last sentence should read:
The BCO cuts out both service and emergency air brakes.
To elaborate, the propulsion and braking control package (a
microprocessor unit) has two outputs to the Apply and Release
magnet valves. These sound similar to SMEE parts, but they are
not trainline. They set up the service brake cylinder pressure.
There is no lockout magnet valve. There is an emergency relay
valve which hangs off the Brake Pipe. When the BP is charged, air
from the A&R valves is relayed to the brake cylinders. In emergency,
the A&R valves are disregarded and the valve pneumatically relays
emergency cylinder pressure to the cylinders.
The SBCO cuts out the brakes at the outlet of the A&R magnet valves
only. It is used when the electronic brake control package has
gone west and won't release the brakes. Using just the SBCO has
no effect on emergency braking. The BCO cock {hhh} blocks off
all air to the cylinders and vents them, cutting out all air brakes.
This would be used in the event of a brake pipe rupture.
The PBCO is a separate device which mechanically retracts the
spring-applied parking brake. If the car has a main reservoir
rupture or otherwise has lost its air, these need to be used
to release the brakes.