Found an interesting article on the Hyro-Boost system that may help someone or all of us when the time comes. The web page with pictures
http://www.babcox.com/editorial/bf/bf030542.htm.
Here is the literature, guess it would not insert pictures here.
Operation, Diagnosis and Repair of HYDRO-BOOST Power Assist Systems
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Bill Williams, Technical Contributor
Hydro-boost power assist was introduced in 1973 by Bendix as an alternative to the vacuum booster. The hydro-boost uses the hydraulic pressure from the power steering system to provide the driver assist in applying the brakes. There are three reasons why a vehicle may be equipped with hydro-boost instead of a vacuum booster:
The is no vacuum source available, as in diesel engines, or the vacuum source available is too weak to adequately supply a vacuum booster.
There is limited space available for the power assist device.
The vehicle requires more assist than is available from a vacuum booster.
Hydro-boost has been used on a variety of vehicles since its introduction and OEM applications presently include:
* 1996 and newer Cobras and all V8 Mustang;
* Most 2500 and 3500 series GMC/Chevy trucks;
* Dodge Ram equipped with Cummins turbo diesel;
* Ford Super Duty series trucks equipped with Diesel; and
* All Hummers - including the H2.
Proper diagnosis of hydro-boost related problems requires an understanding of how the system works. A typical hydro-boost is shown in Figure 1. The hydro-boost is plumbed in line with the steering gear. The power steering pump supplies pressurized fluid for both the power steering gear and hydro-boost.
Spool Valve
Fluid flow in and out of the hydro-boost is controlled by what is known as a spool valve. Spool valves are used in a variety of hydraulic components, such as the valve body of an automatic transmission. A spool valve is basically a hollow cylinder with a number of rings machined into it (see Figure 2). The surface of the spool valve is highly polished to form a sealing surface. The raised portions of the cylinder are called lands while the indentations are called annular grooves.
Figure 3 shows a simplified spool valve positioned in a bore with three ports. The path of the pressurized fluid from port 1 is determined by the position of the spool valve. The spool valve is positioned in figure 4 to allow fluid flow from port 1 to port 2 while port 3 is blocked by land #1. Figure 4 shows the spool valve moved to the left which changes the fluid flow. The fluid flow is now from port 1 to port 3 with port 2 being blocked by land #2. The spool valve in a hydro-boost works in a similar fashion.
Hydro-boost Construction
Figure 5 shows a cutaway of a hydro-boost with all the major components labeled. These include the housing, power chamber, input rod assembly, a lever assembly, a power piston, spool valve assembly and an output rod. The housing is fitted with three ports as identified in Figure 6. The spool valve fits into a precisely machined bore that is part of the hydro-boost housing as shown in Figure 7. The fit between the spool valve and the bore is such that it creates a seal while at the same time allowing enough fluid between the lands and bore to provide lubrication. The spool valve's position is determined by the lever assembly which is connected to the input rod.
Pedal Unapplied
When the brakes are unapplied the spool valve is positioned as shown in Figure 8. In this position the pressurized fluid from the power steering pump is allowed to flow to the steering gear, but not into the power chamber. The spool valve vents the power chamber to the return line of the power steering pump reservoir.
Pedal Applied
Once the brake is applied, the input rod moves forward (left) to the power piston assembly. The piston return spring is preventing the power piston and pin "A" from moving forward. This lack of movement causes the input rod to force the travel limiter valve assembly into the power piston which results in pin "B" moving forward. The lack of movement at pin "A" and forward movement of pin "B" causes the lever to pivot at pin "A". The top of the lever moves forward (left) which results in moving the spool valve (See Figure 9).
The forward movement of the spool valve closes the reservoir port which seals off the power chamber. Continued movement of the spool valve opens the pressure port from the power steering pump allowing pressurized fluid into the power chamber while at the same time maintaining the fluid flow to the steering gear. The pressure in the power chamber causes the power piston to move forward (left) which applies the brakes through the output rod (See Figure 10).
Pedal Released
Once the brakes are released, the spool valve return spring pushes the spool valve back to its rest position. This vents the pressure in the power chamber to the power steering pump reservoir through the return line. The power piston and lever assembly are returned to their rest position by return springs which in turn brings the brake pedal back to an unapplied position.
Back-Up
Similar to the vacuum booster, the hydro-boost is equipped with a backup or reserve in case the source of pressurized fluid is lost. A failure in the power steering system, such as a broken hose, broken power steering pump drive belt, or failed pump, would result in a loss of pressure to both the hydro-boost and steering gear. The hydro-boost uses a high-pressure accumulator to store power steering fluid under pressure in the event of a failure. There are two types of accumulators used, some hydro-boost units use an external accumulator, while others incorporate the accumulator in the power piston. The accumulator could be either of the spring-loaded variety or nitrogen-gas type.
In the event of a loss of pressurized fluid, the accumulator will provide two to three power assisted stops. Upon the first application of the brakes after an engine stall or loss of power steering, you would find approximately 60 to 75% of the normal assist available. If you were to release and apply the brakes again, you would find approximately 30% to 40% assist, then again approximately 10% to 20%, until you have depleted all stored reserve assist. Once you have depleted all of the stored pressure, the brakes will no longer have power assist and will be manual in their operations.
During normal operation, the accumulator is charged by pump pressure though a check valve assembly (See Figure 11). The check valve allows fluid into the accumulator, but prevents it from escaping. When the pressure in the power chamber is lost due to a failure, the input rod linkage will override the power piston linkage and cause the check valve to be opened. The open check valve will release the stored fluid in the accumulator into the power chamber which will provide the power assist.
Pedal Feel
The hydro-boost generates a different pedal feel than a vacuum booster. Basic function can be checked by pumping the brake pedal until hard with the engine off and then starting the engine while maintaining slight pedal pressure. Proper operation should result in the brake pedal sinking down and then pushing back up against your foot. The sinking of the pedal when the engine was started is a result of the power chamber being pressurized. Once the power steering system is at full pressure, it results in the pedal pushing back against your foot pressure.
Diagnosis
The hydro-boost requires a constant source of pressurized power steering fluid for proper operation. Problems in the power steering system will be reflected in the operation of the hydro-boost.
Accurate diagnosis of a hydro-boost power assist device depends on combining an understanding of their function with a logical diagnostic approach. Hydro-boost problems usually break down into the following categories:
* Noise problems; * Slow or incomplete pedal return; * Over sensitive braking; and * Self applying brakes Note: Each of these is covered in the following sections. Noisy Booster
Confirm complaint making sure to note type of noise and when it occurs. If the noise occurs during high brake pedal efforts or quick pedal release, go to step 2. If the noise occurs during low brake pedal effort, engine idle - no pedal effort, or normal driving conditions, go to step 3.
Noise occurs during high brake pedal efforts or quick pedal release: See normal operation noises listed below. Most likely the noises being heard are normal based on the type of conditions.
Operate the vehicle until the engine is at normal operating temperature. Duplicate the operating conditions above and listen for the noise.
Compare results with the normal noises listed under the more info section and with a known acceptable system.
Normal Operation Noises
Properly operating hydro-boost units will produce certain noises. These noises occur, for the most part when the brake pedal is manipulated in a manner not associated with everyday driving habits. The general categories of normal operating noises are (1) hissing noises and (2) clunk/clatter noises.
Linear Mode
