Air Brake Pressure Balance Test
Bendix Commercial Vehicle Systems recommended the following procedure to determine the air pressure differences between the various axles of a single or combination vehicle. For the test, use a dual test gauge with a 0 to 150 psi range.
First, perform vehicle preparation and place the vehicle dash control valves in the normal over-the-road position.
If the vehicle being tested is a single unit, install one half of the dual test gauge directly in the number one or primary circuit delivery of the brake valve. For tractors or combination vehicles, install one half of the dual test gauge in the trailer service (control) line between the tractor and trailer. Note that this half of the test gauge will remain there for the duration of the test.
Install the other half of the test gauge in a service brake chamber at axle number one. Repeat the procedure at each axle of the vehicle combination. Note that the test gauge connection must be between the air chamber and any control valve, ideally directly in or at the chamber.
Next, gradually apply the brakes via the brake valve and record the axle pressure in the increasing column of axle number one when the brake valve primary delivery or trailer service control line pressure is at 5, 10, 15, 20, 25, 30, 40, 50 and 60 psi. Repeat the procedure for each axle of the vehicle combination and record pressure readings on the chart. (See Figure 1).
Readings during this phase of the procedure should be taken with increasing air pressure only. Do not over apply and then release, as an inaccurate reading will occur. Release the brakes fully and reapply if you have over-applied pressure.
Repeat the procedure using decreasing air pressure. Fully apply the service brakes, then gradually release and record the axle pressure in the decreasing column of the appropriate axle when the brake valve primary delivery or the trailer service line pressure is at 60, 50, 40, 30, 25, 20, 15, 10 and 5 respectively. Repeat the procedure for each axle and record the pressure readings on the chart.
