You must mean that you lose steering boost when turning....you cant lose your steering because there is a solid link between the steering wheel and the steering box......unless something is slipping in the system. If something slipped, your steering wheel would be way off so that's not happening.

Hydroboost in those years had some complaints that the boost went away when the pump drive belt got wet and slipped during high pump output situations. Turning while braking does put a load on the pump drive....have you checked your belt to see if it is (1)the proper belt for the application and not bottoming in the pulley and (2) not glazed or worn nor loose? Also, if your engine went into a really low idle speed situation, the pump output might be affected.

Next, the pump may be shot and not capable of sufficient output. Lastly, does it have the right amount of fluid?