I have a 1966 mustang that I put a power brake booster and dual master cylinder on, it has drum brakes. The problem I have is the brake pedal seems to have no pressure at all. The brakes work but the pedal goes to the floor very easily. I have bled the brakes several times with the same results. I read the latest article on installing a power booster in the latest issue of "Mustang Monthly", and it states that if the push rod from the booster is too long it will not be possible to bleed the brakes. I would appreciate any comments or confirmations on this topic.  

If you haven't bench bleed the MC it needs to come out and be bled.  Also drum brake shoe adjustment have a lot to do with the height of the brake pedal and the MC input rod dimension is also critical for getting getting correct brake activation.  The issue that MM was refering to causes the MC internal shuttle valve to improperly displace.  

what jlg said and also from what i have read up on i think i'm not sure buti think you also have to go with a shorter break pedal. thats going by what the article in mm has said.