so go figger....I been using DOT 5 in this car since I had it...16 years now....
I used DOT 5 (silicone) in my 68. I just used gravity bleeding and got a pretty good pedal. It was a little bit soft however, but still very driveable. I took my 68 to Guldstrand Motorsports for an alighnment and asked them to bleed the brakes. I've read complaints were people have said that silicone produces a soft pedal. Not the case here now, stepping onto the brake pedal of my DOT 5 68 is like stepping onto a rock.
Before switching to DOT 5, everything in my brake system was all new and all dry; MC, SS brake lines and new brass distribution junctions, new MC distribution fitting, all new SSBC aluminum brake calipers.