My friend Ryan and I just finished upgrading the brakes on my 1960 Buick invicta and have run into some issues. Some background first. The master and booster where installed using a new 1968 LeSabre master , booster and proportioning valve that was removed from a stock 68 wildcat. After all the lines were hooked up, we bled the system using the standard start at the right rear wheel. Then we found out that according to the manual, you need to start with the left front and go the opposite direction. So we did that. The issue we are confronted with is a soft pedal when the engine is running. Sitting still without the engine on, the pedal seems perfect and right where it shoudl be with a good feel. Once the engine is turned on, the pedal softly goes to the floor. Perhaps we still have air in the brake line? It was suggested that we should bleed with the engine running to affect the booster. I've never heard of that method. Comments or suggestions would be appreciated.