Bendix Commercial Vehicle Systems AIR PRESSURE BALANCE PRESSURE TESTS - Manual (Page 3)

VEHICLE PREPARATION FOR TESTING
1. Adjust all brakes equally using the vehicle
manufacturer’s procedure.
2. Secure the vehicle on a level surface by means other
than the brakes and charge the air system to governor
cut-out (approximately 120 psi). Shut off engine.
CAUTION: To avoid possible injury due to
automatic application of the parking brakes during
testing, it is recommended that the parking brakes
be mechanically caged.
3. Check the vehicle or combination for excessive air
system leakage. If the combined leakage exceeds the
limits below, repair the sources of leakage before
proceeding.
AIR BRAKE PRESSURE BALANCE TEST
Objective: To determine the air pressure differences
between the various axles of a single or
combination vehicle.
Equipment: Dual test gauge (use a gauge with a 0-150
psi range.)
Testing:
1. Perform vehicle preparation and place the vehicle dash
control valves in the normal over-the-road position.
2. If the vehicle being tested is a single unit, such as a
straight truck or bus, install one half of the dual test
gauge directly in the #1 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 tractor and trailer).
NOTE: This half of the test gauge will remain here
for the duration of the test. Install the other half of
the test gauge in a service brake chamber at axle #1.
(Procedure will be repeated at each axle of the vehicle
combination). NOTE: Test gauge connection must
be between the air chamber and any control valve,
ideally directly in or at the chamber.
Brakes Brakes Fully
Released Applied
Single
Vehicle 4 psi in 2 mins. 6 psi in 2 mins.
Tractor
w/Trailer 6 psi in 2 mins. 8 psi in 2 mins.
Tractor
w/2 Trailers 8 psi in 2 mins. 10 psi in 2 mins.
3. Gradually apply the brakes via the brake valve and
record the axle pressure in the increasing (INC) column
of the axle #1 when brake valve primary delivery or
trailer service (control) line pressure is at 5, 10, 15,
20, 25, 30, 40, 50 and 60 psi. Repeat procedure for
each axle of the vehicle combination and record
pressure readings on the chart. (See partial example.)
IMPORTANT: 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 brakes fully
and re-apply, if pressure is over applied.
4. Repeat the procedure using decreasing air pressure.
Fully apply (75 psi application min.) the service brakes
then gradually release and record the axle pressure
in the decreasing (DEC) column of the appropriate
axle when brake valve primary delivery or trailer
service (control) 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.
EXAMPLE:
IMPORTANT: Readings during this phase of the
procedure should be taken with decreasing air
pressure only. Do not under shoot the pressure and
partially reapply pressure as an inaccurate reading
will result. Fully apply the brakes and exhaust down
to the specified pressure.
5. When testing is complete, uncage the parking brakes
unless you are proceeding to FOUNDATION BRAKE
THRESHOLD PRESSURE TESTING.
Air Brake Pressure Balance Test