hydroboost problems

[Prophet455]

I am just getting the project up and running. Using a type 2 Saginaw pump at a 1:1 ratio feeding from a PSC reservoir to power a hydroboost and stock second gen f body type steering box. All parts are new or rebuilt. When I step on the brake, the drag will slip the belt until it kills the motor. The harder you push the faster itll kill the engine. Not sure where to look for the issue- is it a bad pump or the hydroboost that has a serious issue? been over line routing and such several times to make sure an obvious dumb mistake should not be the issue.
Obviously driving it is not an option- all testing and work is done sitting in the garage. What ideas do you guys have and how to test?
thanks