Spicer Tandem Drive Axles Service Manual D__R440, 460, 480, D_R461, 462, 463, 521, 581, 601, 651, 652, 653 - Manual (Page 132)

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Inter-Axle Differential Lockout
Quick Release Valve
If quick release valve fails to operate properly, it may be re-
paired as follows:
1. Disassemble valve.
2. Inspect valve body, valve seat and spring (if used) for
evidence of faulty operation.
3. Replace faulty parts, then reassemble valve.
Pressure Switch
Replace pressure switch as an assembly.