It sounds like you have identified your problem. The power steering pump provides fluid to your gearbox and hydro boost. When you apply brakes, the steering becomes more difficult b/c your pump is supplying a fixed amount of fluid to your gearbox and hydro boost at idle. When you raise your RPMs it flows more fluid and voila your steering works better.

This should not be an issue with a stock truck when everything is functioning correctly. Since you have replaced your pump and gearbox, the next logical part is the hydro boost as you have mentioned however, they are not cheap.

Have you noticed any fluid leaking from the hydro boost, inside the cab where the plunger enters it?

Have you bled everything sufficiently to make sure there is no air trapped in the system?

What type of fluid did you fill the system back up with?

The factory puts ATF in the power steering pump. Since you have changed out the pump and gearbox, you have essentially drained the entire system (with the exception of the HB)

 

I have the same problem. It does not happen all the time, but like you said I have to hit the peddle to give it power to turn. Brakes are great, but at a stop it is getting really hard to turn the wheel. I flushed the entire system and ran new ATF through it to clean it out and refill it. I kinda quieted the power steering pump down so I dont think it is the pump, I am kinda leaning towards the hydroboost like you are thinking.

 

OK, not a hydroboost. Warrantied gear box. Still the same. Put a FORD pump on it. Still the same. It doesnt get better if I two foot it. I give up

 

Are you sure you've got all the air out of system?

 

Check out the for sale stuff I think there's a used hyrdoboost unit for 100 bucks I think


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