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The actual Freightliner reservoir number , if not available from F/L , could be replaced with any P/S reservoir of a similar capacity . Fabricating mounts and adapting hose fittings would be required .
Have you looked at ; Dorman ; products , available on Amazon , that's where I got a coolant reservoir for my side rad F/L chassis .
EDIT: Is this the reservoir you need ? Red box , yellow arrow is coolant reservoir.