I have an 86 5.0 LX Conv. and need to replace the master cylinder but don't know if i have power or standard brakes.

Look behind your master cyl. There should be a 6" dia (approx) canister, black in color, with a vacuum line running to it. if not, you do not have power brakes. if so, then you do have power brakes.

Unless someone covnerted it, it has power brakes. Only the very early models had manual brakes. But like Tony said, cehck for the booster behind the MC to be sure.