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The bit I don't get is this:
Driving along COV1 has pressure on both its inputs. Port D has pressure via the handbrake valve and port E has pressure from the foot brake circuit.
When the driver applies the handbrake port D is vented causing COV1 to take its input from port E and the brake stays OFF, still pressurised. I know this is not what really happens but I don't see it works. Port B has to be vented for the spring brake to apply.
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