I have a 1999 Silverado 2500 4X4. The steering box was leaking so I bought a refurbished box and installed it yesterday. When I started the truck up after the new box was installed the power steering and breaks were working. I took the truck for a test drive and the power steering stopped working and the break boost is not working either. The pump is new I put it on a few months ago. I checked that the pump was pumping at the point it connects to the master cylinder and there is fluid there. I also checked where the pressure line comes out the master cylinder to the gear box and there is fluid there. What more can I check before I take the gear box back and get a different one? Just want to make sure I am replacing the proper part.

 

Unfortunately, your 1999 K-2500 has the now discontinued GM "Hydroboost" system that replaced the vacuum booster brake assembly. I am not all that familiar with it since I haven't worked on that system, but I can see how it works easily enough. Hopefully, this video will help you out. I believe that there is a kit available to return the system back to the vacuum booster brake assist system, but I am sure it's likely not cheap.