Hydroboost Hell

[arkbuilder]

We are having serious cavitation issues with a hydroboost system that is installed on a 1972 Chevelle. The car was recently upgraded with a March serpentine system which utilizes the GM II mini PS pump. We have read that this pump may have compatibilities with a hydroboost system so we have checked all of the line sizes and ended up changing the return line from the reservoir to a 1/2" line and increasing the size of the reservoir. We then carefully bleed the system with the car off and then go out for a drive. Everything is fine for 10 min or so and then we get the death purr and the system starts to cavitate. Where the hell is the air coming from?? Any ideas?? No leaking lines anywhere in the system..