Air and Vacuum Brake Systems

PURPOSE: This rule describes the procedures and standards for the inspection of air and vacuum brake systems. The inspection of brakes is a requirement of section 307.365, RSMo. (1) Vehicles With a Vacuum Brake System. (A) On vehicles equipped with a vacuum brake system, the engine should be allowed to run for one (1) minute to build vacuum. Inspect system for collapsed, broken, badly chafed and improperly supported hoses, tubes and loose or broken hose clamps. Depress brake pedal with moderate foot force. While maintaining this force on the pedal, start engine and observe if pedal falls slightly when engine starts. Build full vacuum, then shut off engine and make as many full brake applications as possible. On vehicles with low vacuum indicators, build full vacuum. Shut off engine and reduce vacuum by making a series of moderate brake applications. A flashing or buzzing signal should function when vacuum reaches eight inches (8") mercury. 1. Reject vehicle if— A. Hoses or tubes are collapsed, leaking, broken, badly chafed, improperly supported or loose because of broken clamps; B. Service brake pedal does not fall slightly as engine is started while pressure is maintained on pedal; C. Vacuum reserve is insufficient to make one (1) full application after engine shutoff; D. Indicator fails to function when system is reduced to eight inches (8") mercury. (2) Vehicles With an Air Brake System. (A) The following inspection procedures apply to all motor vehicles equipped by the manufacturer with an air brake system: 1. With the engine running, ensure the brake system’s air pressure is ninety to one hundred pounds per square inch (90– 100 psi); 2. Shut the engine off and place the key in the auxiliary position; 3. Make one (1) brake and hold application to observe whether air pressure is maintained; 4. Complete a series of brake applications until the lowpressure warning light and/or buzzer operates. Continue brake applications until reaching fifty pounds per square inch (50 psi); 5. Start the engine and observe if the time to build air pressure from fifty to ninety pounds per square inch (50 to 90 psi) exceeds five (5) minutes; 6. Ensure the brake system reaches a full charge without exceeding one hundred twenty-five pounds per square inch (125 psi); 7. Inspect all air hoses and connections; 8. Inspect lines for contact with frame, axles, or exhaust system; 9. Inspect for leaks that may be heard from valves, diaphragms, and piston cups; 10. Inspect air safety valve; and 11. Inspect condition of compressor drive belts. (B) Reject vehicle if— 1. Time required to build pressure from fifty to ninety (50– 90) psi exceeds five (5) minutes; 2. Low pressure warning light and/or buzzer fails to function when pressure is lowered to between fifty to seventy (50–70) psi; 3. Governor cut-out pressure exceeds one hundred twentyfive (125) psi; 4. Governor cut-in pressure is lower than eighty (80) psi; 5. Leakage in psi per minute exceeds three (3) psi for single vehicles and four (4) psi for combination of two (2) vehicles with brakes applied; 6. Leakage in psi per minute exceeds two (2) psi for single vehicles and three (3) psi for combination of two (2) vehicles with brakes released; 7. Air reserve is not sufficient to permit one (1) full brake application after engine is stopped; 8. Reservoir pressure is lowered more than twenty percent (20%) of first reading; 9. Leaks, breaks, crimps, or cracks in the air hoses, tubes, or connections are present; 10. Lines are being rubbed by moving parts or are touching the exhaust system; 11. Leaks in valves, diaphragms, or piston cups can be heard; 12. Air safety valve does not operate; or 13. Compressor drive belts are badly worn or loose. (3) Air Brakes Adjustment. (A) On vehicles equipped with cam brakes, mark each brake chamber push rod at the face of the brake chamber with the brakes released. Apply the air brakes fully, minimum air pressure eighty-five (85) psi, and measure the distance the push rod travels from the face of the chamber to the mark previously made when the brakes were released. This measurement is the push-rod stroke and is illustrated in Figure 1: (B) Reject vehicle if the push-rod travel exceeds the maximum stroke listed in the following table: HIGHWAY PATROL HIGHWAY PATROL (4) Commercial Vehicles Emergency and Parking Brakes. (A) Every vehicle equipped exclusively with an air brake system and every commercial motor vehicle equipped with a vacuum, air over hydraulic, hydroboost, or electrohydraulic booster brake system shall be equipped with parking brakes adequate to hold the vehicle or combination on any grade upon which it is operated under any condition of loading, on a surface free from ice and snow. The parking brake shall be capable of being applied by the driver’s muscular effort or by spring action. Their operation may be assisted by the service brakes or other source of power, provided that failure of the service brakes or other power assisting mechanisms will not prevent the parking brakes from being applied. The parking brake shall be so designed that when once applied it shall remain in the applied position despite exhaustion of any source of energy or leakage of any kind. Emergency system— apply the emergency operating control fully or release air pressure from the spring brake actuators using the manual control valve. Observe locking and holding feature of the actuating mechanism. Observe operating mechanisms for bottoming before brakes are fully applied. Observe if spring brakes apply when control valve is manually operated. Inspect for worn, missing, or defective cotter pins, springs, rods, yokes, couplings, or anchor pins and cables. Observe if mechanism releases brakes when release control is operated. (B) Reject vehicle if— 1. Operating mechanisms fail to hold brakes in applied position without manual effort; 2. Operating mechanism bottoms before brakes are fully applied; 3. Spring brakes fail to apply when control valve is operated; 4. Mechanical parts are missing, broken or badly worn or pull cables are badly worn, stretched, frayed or not operating freely; or 5. Brakes do not fully release when release control is operated. AUTHORITY: section 307.360, RSMo 2016.* Original rule filed Nov. 4, 1968, effective Nov. 14, 1968. Amended: Filed Dec. 5, 1969, effective Dec. 15, 1969. Amended: Filed March 9, 1970, effective March 19, 1970. Amended: Filed Nov. 9, 1971, effective Nov. 19, 1971. Amended: Filed May 21, 1974, effective May 31, 1974. Amended: Filed Dec. 1, 1975, effective Dec. 11, 1975. Amended: Filed March 3, 1980, effective June 12, 1980. Amended: Filed April 2, 1992, effective Sept. 6, 1992. Amended: Filed June 2, 1993, effective Nov. 8, 1993. Emergency rescission and rule filed May 15, 1997, effective June 16, 1997, expired Nov. 30, 1997. Rescinded and readopted: Filed May 15, 1997, effective Nov. 30, 1997. Amended: Filed Nov. 1, 2001, effective April 30, 2002. Amended: Filed March 16, 2022, effective Oct. 30, 2022. *Original authority: 307.360, RSMo 1967, amended 1971, 1973, 1979, 1999.