Power Brake Conversion

[Reece71]

Our 71 C10 had manual brakes - disc/drum. We decided to covert to power brakes and got a conversion kit from Brothers. It had the booster, master cylinder and proportioning valve all together. We bled the master cylinder, installed it, connected the vacuum to the carb, bled the brakes and took it for a test. The pedal is extremely hard - you pretty much have to stand on it to get it to stop. It's like the booster is not working. Got plenty of vacuum, the check valve is working. I don't know what else I can check. My son and I have been working on this project for a few months and tomorrow is his last day of school is tomorrow and he is dying to drive it before he is out for the summer. Thanks for any ideas.