EL-740 Air Compressor Service Procedure

Air inlet filters should be cleaned or replaced when the vacuum level exceeds 1.2 kPa (20 inches of H2O) at compression. The governor settings and the unloader operation should be checked regularly. Maximum desirable reservoir pressure for highway brake use is 861 kPa (125 psi). Recommended pressure range is 103 to 207 kPa (15 to 30 psi).

Unloader components are serviced by removing the unloader assembly from the top of the air compressor head. Because of Midland’s “valve-in-head” design, servicing the EL-740 Air Compressor for other than ring or bearing wear is accomplished by servicing or replacing the head assembly only. Inlet and exhaust valves are serviced in the same procedure.


(1) Unloader body.
(2) Expander spring.
(3) Diaphragm.
(4) Disc.
(5) Retainer ring.
(6) Bushing.
(7) Unloader pin spring.
(8) Unloader pin.

Exhaust Valve

(1) Valve seat.
(2) Copper washer.
(3) Valve.
(4) Spring.
(5) Exhaust valve stem.

Inlet Valve

If the cylinder head can not be rebuilt, replacement heads are available. The following replacement cylinder heads are available: Air Cooled-Threaded Port Type, Air Cooled-Manifold Type, Water Cooled-Threaded Port Type and Water Cooled-Manifold Type.

If an air compressor is in need of piston rings, bearings and seals, repair kits are available or a factory serviced air compressor can be installed. Pistons and rings are available in standard, 0.25 mm (.010 in) and 0.50 mm (.020 in) oversize. Bearing insert kits are available in standard, 0.25 mm (.010 in), 0.50 mm (.020 in) and 0.75 mm (.030 in) undersize.


1. Oversize cylinder blocks must be used in conjunction with oversize pistons.

2. Cylinder blocks that have been rebored oversize should be honed to ensure proper piston ring seating.

3. Air compressors that require new ring sets but do not require reboring of the cylinder block should have cylinder bores re-honed to a crosshatch pattern to ensure proper ring seating.

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