I agree with the Air in the Brake System if the Brake Fluid in the Reservoir is low.
If both chambers were full previously and still full I would look for external Leaks. If none look behind the Master Cylinder and see if fluid has leaked into the Brake Booster.

If the both chambers of Brake Fluid Reservoir are Full I would be thinking the Master Cylinder is no good.

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84 300D, 82 Volvo 244Gl Diesel