brake master cyl switch HELP



I have a 1969 fj40 and switched the single resuivoiur cylinder to a double out of a 78 fj40, i simply cut out some of the brace on the firewall and drilled four holes to mount it. every thing works great i can almost lock up the 32's i have without the booster, (booster won't fit, yet...) anyway i have had three master cylinders (two china rebuilds and one toyota origional) all end up leaking from the rear in a month or so. Does anyone know why? or a reason thaat you cannot use the double cylinder without a booster? Thanks for any help.