First, a well functioning vacuum booster can generate up to 1200+PSI~, so we're not all to far past that point. The power source for the hydraulic brake assist system is the PS pump, so detuning the PS pump for gallon per minute flow rate (GPM) is what the Heidt's valve does. This is a step in the right direction when it is felt that a hydraulic brake assist unit is providing too much response, though the max pressure relief setting in the PS pump is also a factor (and is separate from GPM). If your PS pump is for instance pumping out 3.5 GPM and backing that up with 1500 PSI max pressure relief, this will provide the brake assist unit with a VERY STRONG power source. Hitting a vacuum booster with a constant source of 26" of vacuum is a comparable scenario. Conventional wisdoms accept that a vacuum booster will not provide much assist when a big cam has been installed, providing 10-13" of vacuum resulting in hard pedal, and that is when the vacuum reserve tanks and pumps are applied to artificially supply a higher power level to the vacuum booster. Spoken the other way around, if you have a stock late 60's / early 70's Saginaw PS pump on say your Chevelle, you are looking at typically 2.2 GPM and 1100 PSI max pressure relief. Under this circumstance with this stock pump, the hydraulic brake assist unit will not have as much amplification as compared to 3.5 GPM and 1500 PSI max pressure relief. These systems will actually run down as low as 850-900 PSI max pressure relief, though I feel they then become somewhat underpowered and can impact steering operations in tight parking lot maneuvers if you have to brake and steer hard into a parking spot (as now we are on the flip side of the discussion in not having enough power from the PS pump).
The installation of the Heidt's PS valve between the PS pump output and the brake assist unit will allow for detuning the actions of the hydraulic brake assist unit. With the steering gear downstream of the brake unit, and the PS flow control valve being installed right after the PS pump / before the brake assist unit, this will also impact the steering (as the steering gear will also see reduced pressures / volumes). If you are for instance pleased with the brake assist, BUT want to knock the assist down in the steering, you would then alternately install the Heidt's PS valve in the second high pressure line that comes out of the brake unit that feeds the steering gear. This way the brake unit operations will be unaffected, though you can now blow off the pressures / volumes available to the steering gear. Note that even though the Heidt's valve is capable of bleeding off fluid back to the PS pump, it still doesn't provide a linear control of the PS pump max pressure relief.
The other aspects of this type of discussion is to look at all of the other variables. What type of vehicle? (which will determine the brake pedal mechanical ratio) What type / bore size of master cylinder is being run? The larger the MC bore size, the firmer the pedal and less line pressure and vice versa. The type of brakes at the wheels is also a variable, as some types (like the C4 Vette based designs) require high line pressures to really function well, while others do not. I would also like to point out that certain master cylinders go into more than just MC bore size discussion, as some are also biased differently. The new design Wilwood 260-8556P for instance is listed as providing a 75 front / 25 rear brake general hydraulic bias. The classic stock C3 Vette MC is closer to a 65 / 35 general bias. This being stated, the Wilwood MC could generate much larger pressures to the front brakes as compared to the rear brakes in a side by side comparison to the C3 Vette MC in certain scenarios. Do note that there are large variables in the brakes at the wheels that can really change these calculations around - caliper piston bore sizes, caliper piston travel required to acheive clamping force (which eats up volume and affects pressure), brake line flex hose expansion, even wheel bearings and axle shaft deflection all comes into play. If you are getting high pressure readings to the front brakes, you should see what is happening at the rear as it may actually be too low. In this scenario, locking up front brakes well before the rears are fully in the game can occur. This is usually brought about due to mismatched brake combinations front / rear with differing pressure and volume requirements. If your front pressure is way high, yet your rear pressures are way low, you may feel that the hydraulic brake assist unit is too powerful by resulting in premature front wheel lockup during hard braking (as the rears aren't coming in strongly enough).
With all of the possible variables in modified vehicles, there is always much more too it than meets the eye at first glance. Phillip , if you would like to discuss your scenario further here, please provide as much information as possible:
* Vehicle Year / Make / Model
* MC bore size / type
* Our model # of brake assist unit
* The exact specs of your brakes at the wheels (type, size, pad compound)
* What type of proportioning valve
* What type of PS pump (specs if possible)
* What type of steering gear
* Tire sizes / compound front and rear
* Suspension mods (spring rates and shock valving also come into play)
Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
Use? Hydratech & Heidt's Power Steering Valve
Linear Mode
