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Air Brakes - Service and Maintenance

The Air Brake System of a trailer is entirely dependent upon the Air Brake System of the towing vehicle for its air supply and control. Therefore, the Air Brake System of the towing vehicle must be in good condition. Otherwise, it will be impossible to obtain good brake performance on the trailer. Before condemning the Air Brake System on a trailer, always be sure the Air Brake System on the towing vehicle is functioning properly. The following is based on the assumption the tractor Air Brake System is functioning properly.

1. Insufficient Brakes -
   1. Brakes need:
      1. Adjusting
      2. Lubricated
      3. Relined
      4. Replaced - Check that correct brand of brake shoes were used during the last service replacement
   2. Tractor protection valve not in the "normal" position
   3. Faulty relay valve
   4. No trailer air supply
      1. Clogged emergency line
   5. Low air pressure in the air brake system (below 100-105 PSI)
   6. Brake valve delivery pressure in towing vehicle below normal
   7. Restricted hose, line filter or tubing
   8. Wrong size air chambers
   
   
   
2. Brakes Apply Too Slowly-
   1. Brakes need:
      1. Adjusting
      2. Relined
   2. Low air pressure in the air brake system (below 100-105 PSI)
   3. Brake valve delivery pressure in towing vehicle below normal
   4. Restricted hose, line filter or tubing
   5. Excessive leakage with the brakes applied
   6. Faulty relay emergency valve
   
   
   
3. Brakes Release Too Slowly-
   1. Brakes need:
      1. Adjusting
      2. Lubricating
   2. Truck brake rigging binding
   3. Exhause port of relay emergency valve is restricted or plugged
   4. Restricted hose, line filter or tubing
   5. Check the towing vehicle governor
      1. Proper setting at 100-105 PSI
         1. Service (If you are unable to adjust to the proper PSI you will not have adequate air pressure to fully release the trailer brakes)
      2. Defective
         1. Replace
            
            
4. Brakes Do Not Apply-
   1. Connecting hoses to the trailer are crossed
   2. Faulty relay emergency valve
   3. Tractor protection valve not functioning properly or not in normal position
   4. No air pressure in air brake system
   5. Restricted hose, line filter or tubing
   6. Hoses between the tractor and trailer not connected
   7. Spring brakes are caged
      
      
5. Brakes Do Not Release-
   1. Connecting hoses to the trailer are crossed
   2. Brake valve on tow vehicle in the applied position
   3. Truck brake rigging is binding
   4. Check foundation brakes for:
      1. Excessive wear
      2. Lubrication
      3. Missing components
   5. Trailer relay emergency valve in emergency position.
      1. Connect the emergency line of your towing vehicle to the trailer emergency line. DO NOT connect the service line.
         1. Air comes out the service line on the trailer
            1. Replace the relay emergency valve
         2. No air comes out the service line on the trailer
            1. Check the air lines for being:
               1. Plugged
               2. Pinched
               3. Restricted
   6. Restricted hose, line filter or tubing
   7. Tractor protection valve is not functioning properly or not placed in normal position
   8. Check to ensure the slack adjusters do not contact the axle tube when the brakes are released.
      1. If they do, check to see if the air chamber push rods are fully retracted at this time.
         1. If they are fully retracted you are ok
         2. If they are not fully retracted, it may be necessary to replace the air chamber push rode with one slightly longer in order to allow the air chamber push rode to fully retract
            
            
6. Brakes "Grab"-
   1. Grease on the brake lining
      1. Reline the brakes
   2. Towing vehicle brake rigging binding
   3. Faulty relay emergency valve
   4. Check the towing vehicle brake valve for:
      1. Compatibility with the trailer brake valve (trailer is typically at 4 PSI cracking pressure). If the crack pressure is higher on the towing vehicle it will allow the trailer to brake first and cause excessive wear on the trailer brakes and possibly tires.
      2. Brake rigging binding
   5. Faulty brake valve on the towing vehicle
   6. No trailer load
   7. Check that correct brand of brake shoes were used during the last service replacement
      
      
7. Uneven Brakes-
   1. Brakes needed:
      1. Adjusting
      2. Lubricating
      3. Relining
   2. Grease on the brake lining
      1. Reline the brakes
   3. Brake shoe return spring broken
   4. Brake drum out of round
   5. Leaking brake chamber or diaphragm
   6. Restricted hose, line filter or tubing
   7. Check that correct brand of brake shoes were used during the last service replacement
      
      
8. Excessive Leakage With Brakes Released-
   1. Leaking:
      1. Relay emergency valve
      2. Drain valve
      3. Tubing
      4. Hose
      5. Coupling
   2. Hose uncoupled
      
      
9. Excessive Leakage With Brakes Fully Applied-
   1. Faulty relay emergency valve
   2. Leaking
      1. Brake chamber
         1. Diaphragm
      2. Tube
      3. Hose
      4. Coupling
   3. Hose uncoupled
      
      
10. Excessive Leaking With Relay Emergency Valve in Emergency Position-
    1. Faulty relay emergency valve
       
       
11. Excessive Oil and Water Present in the Air Brake System-
    1. Reservoir not drained often enough
    2. Air drier on towing vehicle not operating properly