If You Can't Stop And You Can't steer your 4x4 then you're in for a dangerous ride, so it's a consensus that the steering and the brakes of your 4x4 are the two most important components. Since these systems can be extremely complex, we always recommend taking your steering and brake components to a professional if you are not 100-percent sure you know what you are doing. To simplify things we'll concentrate on a steering and hydroboost combination system. This is found in many heavy-duty trucks, especially those with diesel engines because they do not require vacuum to boost the brakes. We recently had the power-steering pump, box, and hydroboost system rebuilt on one of our project trucks. We also added a hydraulic ram for steering assist. If you've ever been inside any of these parts then you know how intricate they are, and if you haven't been then this will answer some of the mysteries within your hydroboost steering/stopping system.
This is one area we highly recommend using experts on. A mistake in any of these rebuilds could have deadly consequences because unlike a manual transmission or an axle, these parts are what keep your vehicle from becoming an out-of-control 3-ton chunk of steel hurtling down the trail or highway. We're not saying don't attempt to take your steering components apart, but we would recommend having a pro put them back together.
The first order of business was taking the power-steering pump and box to Lee Manufacturing for a rebuild and boost. Lee Manufacturing is unknown in much of the off-road world, but the company has been building specialty steering components for many trophy trucks and desert racers for years. This is a P-style pump where the pump itself seals within its own reservoir as compared to the other commonly used TC-style pumps where there is an external or remote reservoir. | 
The pump consists of the flow-control/piston assembly and the pump itself. From right to left is the flow control, the piston or poppet, and the piston spring. Consider this the door that the pressurized fluid opens and leaves the pump through. The piston floats within the chamber in case the backpressure overwhelms the piston. Backpressure occurs when the steering is turned to full lock and then pulled hard until the pump buzzes. This backpressure is bad for your pump because it recirculates and cooks the fluid. |
Inside the piston is a spring-loaded plunger that seals up against a small ball bearing. This is where the pressure for the pump is determined. As the pump pushes fluid against the ball and plunger, the spring is forcing the plunger against the fluid, causing pressure to build up, similar to the way a spring holding a door shut causes you to exert more pressure to get through the door. | 
After the fluid has increased in pressure from the piston, its volume is controlled by the flow control. Lee Manufacturing bored out the flow control for additional volume since the new steering system will be using a hydraulic ram assist. The pressure line going to the hydroboost and then the steering box screws directly into the flow control on the back of the pump housing. |
The pump has (top to bottom) the thrust plate, pressure plate, ring and rotor with vanes, and then the driveshaft that is keyed into both the rotor and the beltdriven pulley. On the left is the original assembly and the larger parts on the right will increase the oil volume to help with the ram as well as boosting the volume at low engine rpm. This is helpful in rockcrawling since turning big tires requires extra fluid even though the engine is at low rpm. | 
With the ring and rotor in the pump, notice how the vanes move in and out of the slots in the rotor as the rotor spins within the oval ring due to centrifugal force. As the space between two vanes increases, it forms a vacuum and pulls fluid in from the reservoir. Then as the space decreases it compresses the fluid. Then as the rotor turns it passes over an opening where the compressed fluid can escape and is routed into the piston chamber. |
The pump is reassembled with new seals and O-rings installed and a new end plate that makes room for the thicker ring, rotor, and pressure plate. Note that a reservoir from a hydroboost-equipped vehicle has two low-pressure return lines, one from the steering box and one from the hydroboost. | |