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I'm trying to electronically control the service brakes on some construction equipment, in addition to maintaining the normal brake pedal functionality. The disk brakes are controlled by the valve pictured below (Bosch Safim 27043FU.)

Brake Valve Assembly.

  • Input P is pressure (around 33.3Bar).
  • Output S is the return to Tank.
  • The port B1 goes to the disk brakes (around 780-850psi when pressed fully).
  • R1 goes to a nearby brake fluid reservoir.

First, I'm having a hard time understanding how this valve assembly works. It looks like there's a 3-way, 3-position pilot operated proportional valve on the left that somehow provides pressure to a piston on the right, which then pushes out brake fluid to the brakes (and separates hydraulic fluid from brake fluid). However, the piston's (middle) input is tied straight to Tank, so I don't understand how it gets pressurized at all. Also, the Valve's only work port seems to loop around the bottom to its own case? (Note: Sorry, the image is a bit fuzzy - the two horizontal lines coming out of right side of the valve are dotted, so I assume that means pilot lines.)

Second, regardless how how that valve assembly works, I want to add another device that will apply pressure to the same brake line, but is electronically controlled (CAN or analog/PWM voltage), and preferably electrically powered (12V). I was looking into "Electric/Hydraulic Brake Actuators" used for trailers, which, as I understand, are basically electric hydraulic pumps that create brake fluid pressure.

The second possibility is some kind of solenoid-driven proportional relief valve, powered by the same S and P lines as the existing brake valve, but I'm not sure how to separate hydraulic fluid and brake fluid - another piston?

The backup plan is just an electric linear actuator pushing on the current brake pedal.

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