It is also possible that you have some parts with rubber that is not compatible with DOT 5 fluid. When I bought all new parts for my 1954 Suburban's brake system (2002), I traced parts back to the manufacturers and some of them would not certify that their parts would work with DOT 5 fluid, and some simply said their parts were not compatible. Eventually, I found parts that are compatible with DOT 5 fluid and I have a hard pedal and no problems.
