Power disc brake conversion kit troubles.

Ok I put a return spring on it, and it appears that the pedal still doesn’t always return like it’s supposed to. The thing is, this engine doesn’t really make enough vacuum for the power brakes to work that well, and for the most part, the car stops very poorly, to the point to where it’s not really safe to drive. And every once in a while the power brakes seem to kind of work, and that’s when I notice the brake pedal not returning properly and the brakes not releasing all the way until I pull up on the pedal.