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Not being familiar with what valve is on it , does the parts book show any o-rings in the valve ? The plastic could be dried out o-rings from somewhere.
Edit : Have you serviced the air-drier yet?
You should be able to trace the air system part through this TM m1078_TM 9-2320-365-24P.pdf
m1078_TM 9-2320-365-24P for the M1078 series trucks.
The rest of the TM's are here
M1078 TM's