Electro Hydraulic Brakes

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Conventional hydraulic brakes rely on pressure generated in the system when the driver pushes the brake pedal. A further development on this was to incorporate electricals in the hydraulic circuit. On depressing the brake pedal a valve opens and releases pressure to the cylinders at the wheel. Also known as power brakes, these electrohydraulic brakes are a vast improvement on traditional mechanical-hydraulic version of brakes.  


  • It is difficult to pin precise dates of invention and development of electrohydraulic or power brakes.
  • History records that Victor Kliesrath and Caleb Bragg invented a vacuum assisted booster system in the 1920s for brakes used in aeronautics.
  • Westinghouse reportedly developed vacuum brake boosting system sometime between 1927 and 1929 used on Chandler cars.
  • Mercedes, Cadillac, Duesenberg, Lincoln and Cadillac adopted vacuum boosted hydraulic brakes for their vehicles by the early 1930s.
  • The Tincher car was, perhaps, the first to use assisted braking in the form of air brakes way back in 1903.
  • The Pierce Arrow used vacuum power booster for brakes in 1928, developed by Bragg-Kliesrath.


Evolution and Adoption

  • Early variations included the hydrovac and hydroboost systems.
  • In the former, prevalent in the 1940s, depressing the pedal increased pressure to the wheel cylinders that in turn actuated a valve to move a triangular arm, which, on rotation, opened a vacuum valve resulting in the pushing of bellows in the slave cylinder to further increase pressure. Pressure is directly proportional to how hard a foot presses down on the brake pedal.
  • The Hydroboost system developed uses power from the steering pump instead of relying on vacuum. Alternatively an electric motor-pump supplies the required pressure. Here, when the brake pedal is depressed a valve in the master cylinder opens to allow the power system to kick in and supply the required braking power. Hydroboost systems are more compact and use fewer mechanical components, making them more reliable.
  • Vacuum boost make use of vacuum generated in the inlet manifold, using a mix of air and hydraulic technologies to maximize braking pressure.
  • Electrohydraulic does away with use of vacuum and, like the hydroboost, uses a pressure tank to activate master cylinder driven by electrically powered pump. The steering pump or the manifold are not touched and the brake system remains independent of engine connection. Some of General Motor cars produced in 1985 used the electro hydraulic brakes.
  • Daimler and Bosch developed the electro hydraulic brake into the sensotronic brake control sometime in 2001 that employed car computer and sensors to adjust braking pressure at each wheel, driven by a central high pressure reservoir, resulting in better anti-skid and braking.
  • Brake by wire electrohydraulic braking is a further advancement. In this system there are small electric motors located close to the wheels that generate required braking pressure. These motor pumps are controlled by a unit that derives information from sensors fitted to the brake pedal and also the sensors that monitor other operations to adjust braking for best, anti-skid, and output results.



  • Engine driven pump in case of a purely electrohydraulic brake unit
  • Master cylinders
  • Cylinders in braking units at the wheels
  • Hydraulic lines-rigid and flexible
  • Sensors
  • Electronic Control unit

How It Works

The purely electrohydraulic braking system works this way:

  • Electric motor pump in the circuit creates pressure in the hydraulic reservoir.
  • Each wheel brake unit has individually controllable pressure loops made up of inlet, outlet and sensors.
  • Optimal performance is assured by making use of wheel brake cylinder information and the brake pressure control to distribute braking force left and right as well as front and rear resulting in improved stability.
  • Electric motor powers piston in the master cylinder that forces fluid from the reservoir through the lines to the brake calipers.

Apart from the electrohydraulic circuit, the unit also has manual backup in case of any electrical failure of the component or the circuit.


  • Electrohydraulic brakes incorporate sensors and a power driven pump. This reduces effort on the part of the driver.
  • Additionally, sensors incorporated into the system sense the braking pressure applied by the foot, driving conditions, wheels speed and other factors to apply effective braking power.
  • Antiskid enabled electrohydraulic brakes automatically sense if a wheel is locking and modulate braking pressure accordingly to prevent skids.
  • Electro-Hydraulic units are much lighter than vacuum assisted units.
  • Electro- Hydraulics place no burden on the engine or the steering pump
  • Fast response and powerful braking resulting in fewer collisions

As always hydraulic and electrical systems have room for improvement. Better hydraulic fluids can be developed that are capable of resisting moisture and heat.


Maintenance Tips

  • High end electrohydraulic units with sensors are best serviced at company’s service stations. However, a driver can carry out basis maintenance such as checking hydraulic oil level, checking for leaks and checking for brake shoe wear and tear.
  • Always check and replace if necessary, hydraulic oil after the rainy season since hydraulic fluid tends to absorb moisture.


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