Bendix Air Compressor Preventive Maintenance

Air Induction

One of the single most important aspects of compressor preventive maintenance is the induction of clean air. The type and interval of maintenance required will vary depending upon the air induction system used.

The intervals listed under the headings below pertain to typical highway and street operation. More frequent maintenance will be required for operation in dusty or dirty environments.

Fig. 9 Polyurethane Sponge Strainer

Polyurethane Sponge Strainer

Every month, 150 operating hours or 5,000 miles, whichever occurs first, remove and wash all of the parts. The strainer element should be cleaned or replaced. If the element is cleaned, it should be washed in a commercial solvent or a detergent and water solution. The element should be saturated in clean engine oil, then squeezed dry before replacing it in the strainer. Be sure to replace the air strainer gasket if the entire air strainer is removed from the compressor intake.

Fig. 10 Dry Element – Pleated Paper Air Strainer

Dry Element – Pleated Paper Air Strainer

Every two months, 800 operating hours or 20,000 miles whichever occurs first, remove the spring clips from either side of mounting baffle and remove the cover. Replace the pleated paper filter and remount the cleaned cover making sure the filter is in position. Make certain to replace the air strainer gasket if the entire air strainer is removed from the compressor intake.

Fig. 11 Intake Adapter

Intake Adapter

When the engine air cleaner is replaced; Some compressors are fitted with intake adapters which allow the compressor intake to be connected to the engine air cleaner, turbo or supercharger. In this case, the compressor receives a supply of clean air from the engine air cleaner. When the engine air filter is changed, the compressor intake adapter should be checked. If it is loose, remove the intake adapter, clean the strainer plate, if applicable, and replace the intake adapter gasket, and reinstall the adapter securely. Check line connections both at the compressor intake adapter and at the engine or engine air cleaner. Inspect the connecting line for kinks and ruptures and replace it if necessary.

Compressor Cooling

Every six months, 1800 operating hours or 50,000 miles, whichever occurs first, inspect the compressor discharge port, inlet cavity and discharge line for evidence of restrictions and carboning. If excessive buildup is noted, thoroughly clean or replace the affected parts and closely inspect the compressor cooling system. Check all compressor coolant lines for kinks and restrictions to flow. Minimum coolant line size is 9.5 mm (3/8 inch) I.D. Check coolant lines for internal clogging from rust scale. If coolant lines appear suspicious, check the coolant flow and compare to the tabulated technical data present in the back of this manual.

Inspect and clean the external air cooling fins on the cylinder portion of the crankcase. If fins are cracked or broken, replace the compressor.

Inspect the air induction system for restrictions.


Every six months, 1800 operating hours or 50,000 miles, whichever occurs first; check external oil supply and return lines, if applicable, for kinks, bends, or restrictions to flow. Supply lines must be a minimum of 4.8 mm (3/1 6 inch) I.D. and return lines must be a minimum of 12.7 mm (1/2 inch) I.D. Oil return lines should slope as sharply as possible back to the engine crankcase and should have as few fittings and bends as possible. Refer to the tabulated technical data in the back of this manual for oil pressure minimum valves.

Compressor Drive

Every six months, 1800 operating hours or 50,000 miles, whichever occurs first, check for noisy compressor operation. Variations in noise level in conjunction with the compression and unloaded cycles generally indicate loose or worn drive components.

On belt drive compressors check for pulley and belt alignment and tension. Adjust as necessary, paying particular attention not to overtighten belt tension. Check for loose and out of aligned pulleys. Adjust or replace as necessary. Compressor crankshaft keyway damage indicates a loose pulley and often requires compressor replacement. Main bearing failures on belt driven compressors often indicate excessive belt tension.

A thorough inspection, and possible replacement, of drive components should be made at each compressor change. Special attention should be given to drive gears and couplings on compressors which have been operated at high discharge pressures due to a blocked or frozen discharge line.

Check all compressor mounting bolts and retighten evenly as necessary. Check the condition of all compressor mounting brackets, tighten hardware as necessary, and replace if damaged.

Operational Tests

Every three months, 900 operating hours or 25,000 miles whichever occurs first. Vehicles manufactured after the effective date of FMVSS 121, with the minimum required reservoir volume, must have a compressor capable of raising air system pressure from 590 to 700 kPa (85 to 100 psi) in 25 seconds or less. This test is performed with the engine operating at maximum governed speed. The vehicle manufacturer must certify this performance on new vehicles with appropriate allowances for air systems with greater than the minimum required reservoir volume.

Check unloader operation by building system pressure to governor cut-out and note that air compression stops. Reduce system pressure to governor cut-in and note that air compression resumes. If the compressor fails to respond as described, make certain the governor is functioning properly before repairing or replacing the compressor.

Compressor Air Leakage Tests

Compressor leakage tests need not be performed on a regular basis. These tests should be performed when; it is suspected that discharge valve leakage is substantially affecting compressor build-up performance, or when it is suspected that the compressor is cycling between the load and unloaded modes due to unloader piston leakage.

These tests must be performed with vehicle parked on a level surface, the engine not running, the entire air system completely drained to 0 kPa (0 psi) and the inlet check valve detail parts removed, if applicable.

Unloader Piston Leakage

Remove the governor and apply shop air pressure to the 1/8 inch pipe thread unloader port on the governor mounting pad. Listen for the escape of air at the inlet cavity. An audible escape of air should not be detected. If any question exists as to leakage, it is recommended that a genuine unloader kit be installed and the cylinder head retested.

Discharge Valve Leakage

Unloader piston leakage must be repaired before this test is performed.

Leakage past the discharge valve can be detected by removing the discharge line, applying shop air to the unloader mechanism and the discharge port and listening for the escape of air at the compressor inlet cavity. A barely audible escape of air is generally acceptable; however, if there is any question as to the leakage rate, it is recommended that the cylinder head or compressor be removed and repaired or replaced. With shop air still applied at the discharge port, apply a soap solution to the valve stop recess on the top of the head. If leakage is detected, the cylinder head must be repaired or replaced. Only genuine remanufactured compressors or service parts and kits should be used.

1. Always block vehicle wheels. Stop engine when working under a vehicle. Depleting vehicle air system pressure may cause vehicle to roll. Keep hands away from chamber push rods and slack adjusters; they may automatically apply as system pressure drops.

2. Never connect or disconnect a hose or line containing air pressure. It may whip as air escapes. Never remove a component or pipe plug unless you are certain all system pressure has been depleted.

3. Never exceed recommended air pressure and always wear safety glasses when working with air pressure. Never look into air jets or direct them at anyone.

4. Never attempt to disassemble a component until you have read and understand recommended procedures. Some components contain powerful springs and injury can result if not properly disassembled. Use only proper tools and observe all precautions pertaining to use of those tools.

5. Use only genuine replacement parts and components.

A. Only components, devices, mounting and attaching hardware specifically designed for use in air brake systems should be used.

B. Replacement hardware, tubing, hose, fittings, etc. should be of equivalent size, type length, and strength as the original equipment.

C. Make certain that when replacing tubing or hose, all supports, clamps or suspending devices that were originally installed are reinstalled.

6. Devices with stripped threads or damaged parts should be replaced. Repairs requiring machining should not be attempted.

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