It could very well be either the power steering pump or the hydroboost booster unit. Those are the only things that the brake and steering systems have in common. If it's only braking that causes the loss, I'd be inclined to start looking at that. But first, the obvious; have you checked the fluid level in the power steering resevoir? It might be low, or get low when the brakes are used a lot. Run the truck, cycle the steering and brakes a few times, then immediately check the level. If it's low, that might be your issue.