Hydraulic fluid is much less compressible than compressed air. A change to one end of a hose filled with hydraulic fluid instantly affects the other end. Compressed air has to flow through the hose until air pressure at the other end is high enough. For this reason, there can be an air brake lag time of about 0.4 second before the service brakes at the rear of your vehicle start to function. To correct this problem, a separate air tank is usually installed near the rear of the vehicle (closer to the rear brake chambers) along with a relay valve to control when that air is used.