The quick release valve moves to reduce brake lag in the air brake system.
#1) The QUICK RELEASE VALVES quickly releases air from the service brake chambers into the atmosphere after a service brake application is released.
#2) The QUICK RELEASE VALVES quickly releases air from the spring brake chambers when either the tractor parking brake or trailer air supply are pulled out on the dash.
Because air brake systems have a constant air supply, air used to apply the service brakes, or compress the parking brake springs is not reused – it is simply evacuated from the system.
By quickly releasing air into the atmosphere a short distance from the brake chambers, brake lag is reduced.
These valves are within close proximity to the brake chambers so the exhausted air travels a short distance when it is expelled.
Brake lag is the time delay from applying the brakes to when the brakes actually apply; conversely, it is when the brake pedal is released to when the brakes actually release.
As brake application pressure increases, so too does brake lag.
Quick release valves are only one of the number of ways that air brake systems reduces brake lag.