Hydroboost problem

[jborders]

I recently installed a new hydroboost unit and master cylinder from a 1993 2500 truck onto my 68 2wd. 98 percent of the time it works perfectly but every now and then when you are in a parking lot maneuvering and turning the wheels quite a bit you hit the brakes and pedal goes really soft and to the floor. As soon as u step on them again their fine. Upon investigating the problem I've found just sitting in the garage engine running if you turn the wheel lock to lock then hit the brake pedal it will go the floor every time the first time you push it. The power steering pump sounds fine and operates quietly. This is the first hydroboost system I've ever had so its new to me any advice would be appreciated.