You can't run hydroboost with just a PS pump, you have to have a PS box as well. The hydraulic assist has a hi pressure inlet and hi pressure outlet. The hi pressure outlet needs to go to the steering box. You can't just run it as a return. This is a good time to upgrade to both PS and hydroboost braking.
This just isn't true. Basic hydraulics, the hydroboost and the power steering box are both open center systems, which means when you're not cranking on the steering wheel or stepping on the brake the fluid circulates from the pump, through the hydroboost, through the steering box, through the cooler, and back to the reservoir, all under no/minimal pressure.
When you crank the steering wheel, the valve redirects the feed to one side of the assist piston in the box, and the other side to the return. Now the line from the pump, through the hydroboost, and up to the steering box is at whatever pressure is needed to displace the volume in the assist cylinder against the resistance of the steering system. When you hit full lock and the piston bottoms, then the pressure in the system hits whatever maximum set by the bypass regulator in the pump. As soon as you release the steering wheel, the spool valve goes back to open center and fluid resumes circulating at minimum pressure.
Now the reason the hydroboost needs it's own separate return is because it needs a hydraulic pressure differential across it even if the steering is being held a full lock (no fluid circulating through the output line to the steering box, but running at max pressure). So it has it's independent return to ensure it can let fluid leave it's system and return to the reservoir as it actuates. Now when you use the hydroboost, it acts as a diversion in the hydraulic system to the input on the power steering box, so steering assist will be affected by the use of the hydroboost.
The steering box and the hydroboost both use open center valving because the pump is a positive displacement vane pump. It has to move fluid, at whatever pressure necessary up to the relief valve pressure. If the lines were under full pressure all the time, the amount of heat built up in the fluid, wear on the pump, and power consumed from the engine would be huge. That's what happens when you hold the steering at full lock, the relive valve is letting the full fluid volume of the pump bypass at maximum pressure straight from the output of the pump back to the reservoir. That's why the pump whines and the engine loads down. The heat problem is compounded because the pressurized fluid is being dumped straight back into the reservoir, not circulating through the steering box, out the return, through the cooler, and then back to the reservoir like it normally does. That's why pumps get cooked by people holding the steering at full lock for extended periods (like doing donuts in the snow).
Consider the output line on the hydroboost to be the same as the power beyond tap on a loader going to a backhoe. If you remove the backhoe, you just hook the input line to the return line to the reservoir, and the loader will still function normally. If you run hydroboost without a steering box, you hook the output to the return and it will still function normally, because it's just the same as using it for stopping in a straight line without touching the steering wheel if it was equipped with a power steering box.
