92 Astro van lost both power steering and power brake assist. It has the hydro-boost.
The power steering pump is the most likely cause of the problem.
My problem in particular was first off I had a seal leak in the top of the steering box. This caused a little loss of pressure but not much. However moisture got into the system and began the slow process of breakdown.
During freezing weather the hydro-boost seals froze and blew out because of the moisture in the system.
The power steering pump was also completely shot.
My fix (expensive) was to replace the hydro boost with a junk yard used one($200), the steering box with a junk yard used one ($50) and a brand new power steering pump ($110).
I did this all myself to save the cost of labour (4 to 6 hours).
Once installed the brake and power steering came back on instantly.
If you have this problem on your vehicle, here is how to check it.
1)Visually check the entire sytsem for signs of leaking power steering fluid. The fluid resovoir, The top of the steering box, the hydro boost, the power steering pump and the fluid lines.
2)While engine is running, undo the fluid lines one at a time that lead into the booster. Are you getting fluid blasting out at you from any line? If no pump is bad, replace it.
If yes, pump is ok. The CHECK VALVE inside the hydro boost is stuck. Replace hydro boost.
When replacing any of these parts it is always a smart idea to replace the rubber hoses and clamps with new ones.
Hopefully this will help a lot a people with the same problem.