I need some input. My 55 merc still has the 4 drums. The manual single line master had some pedal movement but got very firm with good braking after just a little pedal travel.

I just installed a power booster and dual master. I bled the brakes and now the pedal is rock hard with just slight movement, about an inch or two to full lockup. Pump the brakes and started while holding pedal and it sunk slightly, so i think the boster is working OK. I adjusted the booster rod so the master was not preloaded. I even spaced the master from the booster and got pedal travel until master engaged, then rock hard. Therefore i think it is in the lines or drums.

Any ideas on the problem? And what tactic should i take for troubleshooting?

Thanks