Dot 5 will not cause rubber to deteriorate. Dot 5 is silcone brake fluid and does not draw water into the brake system and line, also dot 5 will not eat your paint like the Dot3,4 and 5.1 fluids.
Yes the rubber boot on the MC may have dried out because of non use or heat and just because of age. If you have left the car i storage from 1980 then I would suggest that you go over the car and check the braking system, antifreese properties, carburation and the entire car. So basically your car has not been driven for over 40+ years. The gas tank may be clogged, the brakes might even be stuck, all kinds of potential issues. The hoses would need to be replaced so you are basically starting a new restoration on the car.
Rich Hartung
desoto1939@aol.com