I have had Aluminum Master Cylinders on my car befor (Olds Cutlasss). I believe they just have an Aluminum Bore as do Aluminum Wheel Cylinders for Drum Brakes.

It is easy to check. Just suhove the Piston/s in manually till you can see the bore or remove the Rear snap-ring and washer (could be a 3 cornered wire retainer spring) and take a peek at the bore. If it has a steel bore it should go all the way to the open end of the Master Cylinder.

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84 300D, 82 Volvo 244Gl Diesel