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Air Brake System Troubleshooting Tests (continued)

TEST 4

Leakage service air delivery

Full pressure, engine stopped, parking brakes

released

1. Make and hold brake application. This can be accomplished

by using the BVA-85

™

brake valve actuator. If the vehicle is

not equipped with a BVA-85

™

brake valve actuator, a block of

wood can be used to hold the foot valve down during these

tests.

2. Allow pressure to stabilize for 1 minute; then begin timing

for 2 minutes while watching the dash gauges for a pressure

drop.

A. Pressure Drop: Single Vehicle (A 4 psi drop within 2

minutes is allowable for either service reservoir)

B. Pressure Drop: Tractor/Trailer (A 6 psi drop within 2

minutes is allowable for either service reservoir)

C. Pressure Drop: Tractor/2 Trailers (An 8 psi drop within 2

minutes is allowable for either service reservoir)

3. Check brake chamber push rod travel (refer to chart for

allowable tolerances). With the parking brakes released and

service brakes applied with 80 to 90 psi of air pressure in the

service chambers.

Brake Chamber Size

Maximum Allowable

Stroke

Maximum Allowable

Stroke - Long Stroke

12 1-3/8" 1-3/4"

16 1-3/4" 2"

20 1-3/4" 2"

24 1-3/4" 2"

24 (max. stroke) - 2½"

30 2" 2½"

TEST 5

Manual Parking Brake Operation

Full pressure, engine idling 600-900 rpm

FOR STRAIGHT TRUCKS, BUSES AND BOBTAIL TRACTOR:

1. Manually operate the park control, yellow button valve, and

note that parking brakes apply and release promptly as the

control valve button is pulled out and pushed in.

FOR TRACTOR/TRAILER COMBINATIONS:

1. Manually operate the tractor protection control valve (trailer

supply valve usually red octagonal button). Note that trailer

brakes apply and release promptly as the control button is

pulled out and pushed in.

2. Manually operate system park control (usually yellow diamond

button) and note all parking brakes (tractor and trailer) apply

promptly.

Make all necessary repairs before proceeding to test 6;

see Checklist 5 for common corrections.

4. Check the angle formed between the brake chamber push

rod and slack adjuster arm. It should be approximately 90° in

the applied position (80-90 psi) and the same across the axle.

Make all necessary repairs before proceeding to test

5; see checklist 4 for common corrections.

CHECKLIST 4

If there is excessive leakage in the service side of

the pneumatic system, one or more of the following

devices could be causing the problem.

NOTE: A leak detector or soap solution will aid in locating the

faulty component.

1. Loose service lines and ﬁ ttings

2. Trailer control valve

3. Stoplight switch

4. Brake chamber diaphragms

5. Tractor protection valve

6. Service brake relay valves

7. Dual brake valve

8. Inverting relay spring brake control valve (where applicable –

usually found on spring brake relay valve) straight trucks and

buses

9. Double check valve.

If the automatic slack adjuster is not adjusting, repair

or replace to obtain desired setting.

CAUTION: If the brake chamber push rod travel

exceeds the allowable stroke, then identify and ﬁ x the

problem. DO NOT adjust and return to service!

Retest to verify proper operation of all items repaired

or replaced.

CHECKLIST 5

If sluggish performance is noted in either test, check

for:

1. Dented or kinked lines

2. Improperly installed hose ﬁ tting

3. A faulty quick release valve or spring brake control valve

4. Binding cam shafts or ungreased cam shaft bushings

If the trailer brakes do not actuate and the trailer supply

line remains charged, check the:

1. Tractor protection control

2. Trailer spring brake valve

3. Binding cam shafts or ungreased cam shaft bushings

Retest to verify proper operation of all items repaired

or replaced.

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BENDIX MC-12 MODULATOR CONTROLLER ASSY Guida alla Risoluzione dei Problemi Pagina 63

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