To properly use the results of a pressure test it is best to first understand the pressures generated during different types of braking and the function of the different brake valves used in the system.
You will need to obtain a brake hydraulic pressure test kit similar to the image above
Because most manufacturers do not publish pressure specifications it is often necessary to use generic values. Typically the hydraulic pressure generated during light to normal braking will be from 300psi to 500psi. Panic braking is generally considered to start at about 600psi. The proportioning valve will start to limit pressure to the rear brakes at this time. If a full panic stop is taking place the proportioning valve will limit the rear brake pressure to about 800psi while the front brakes will climb to between 1500psi to 2000psi. The psi numbers presented here are averages based on many tests performed on different vehicles.
Two types of pressure testing are common, checking the front to rear brake pressures and checking an axleʼs side to side pressures. The uses of each of the tests is explained below:
Front to Rear pressure for a properly operating system
Checking front to rear pressure:
Performing a front to rear pressure test can be done to help diagnose the following conditions:
- Proportioning valve malfunction – Diagnose the cause of rear wheel lockup during panic braking or possibly a proportioning valve causing a restriction (wonʼt reopen after release).
- Premature front brake pad wear – Pressure tests are used to determine if a lack of rear braking is related to the cause of front pad wear.
- Height sensitive control valve diagnosis or adjustment – These valves usually incorporate pressure tests in the adjustment procedure and can be diagnosed using pressure gauges.
Side to Side pressure for a malfunctioning system
Checking side to side pressure:
Performing a side to side pressure test can be done to help diagnose the following conditions:
- Brake pull – different hydraulic pressures results in different clamping forces and can result in a pull
- Side to side pad wear differences – if the pads on one side of an axle are wearing faster than the pads on the opposite side
Performing a Front to Rear Pressure Test
- Connect the gauges using the manufacturerʼs instructions. Most pressure gauges connect to the caliper or wheel cylinderʼs bleeder screw hole (See Figure 85.4). Make sure to properly bleed each gauge hose.
- Place the gaugeʼs memory needles on zero.
- Start the vehicle. Apply the brake first using light to moderate force as if making a normal stop. Note the pressure readings, under this pedal force the front pressures should not be over 500psi in most vehicles. Compare the front and rear pressure readings. At this pedal force they should be equal or near equal. If equal continue to next step. If there is a large difference with the rears being substantially lower than the fronts there is a potential problem. The list of possible causes will depend on the system configuration. The list will include:
- Restricted proportioning valve
- Restricted brake hose to the wheel or axle
- Restricted steel brake line
- Out of adjustment or restricted height sensitive control valve
- Restriction in ABS modulator