Multi-circuit protection valves - features and advantages

A distinction is made between different types of multi-circuit protection valves

  • Multi-circuit protection valves with limited backflow in series or parallel connection, with or without bypass.
  • Multi-circuit protection valves in series connection with one or two integrated pressure limiters and check valves for circuits 3 and 4 and electronic pressure sensors for circuits 1 and 2.


To comply with Directive 98/12/EC, the multi-circuit protection valves are equipped with a bleed-back function.

Multi-circuit protection valves: Important terms briefly explained

  • Opening pressure: pressure required to open the circuits.
  • Closing pressure (stabilizing pressure): Pressure that results in shutdown of the failed circuit. Above the closing pressure, a limited backflow allows pressure equalization of the respective interconnected circuits.
  • Series connection: This means that the auxiliary consumers (circuits 3 and 4) are connected downstream of the main consumers (circuits 1 and 2). A backflow from the auxiliary consumers to the main consumers is not possible due to the integrated check valves.
  • Parallel connection: In parallel connection, all circuits are connected to each other. Accordingly, above the closing pressure, a limited backflow between all circuits, i.e. also from the auxiliary consumers to the main consumers, is normal.
  • Bypasses: These allow priority filling of the circuits equipped with them when the entire system is depressurized. They are often used in circuits 1 and 2. However, in the case of valve modifications in parallel connection, bypasses can also be used in circuits 3 and 4.
  • Bleed-back function: If the pressure in circuit 1 drops due to a defective line, circuit 3 is also drained. When trucks stop, at traffic lights for example, the handbrake is usually applied. If there is no pressure in circuit 3, it will no longer be released, thus preventing the truck from continuing to drive until the air loss has been rectified. If a trailer or semitrailer is attached to a tractor, venting the red air hose initiates braking of the trailer, which results in the same thing.

Multi-circuit protection valves: Legal particularities

... Adaptation Directive 91/422/EEC and Directive 98/12/EC

For vehicles first registered after October 1994, the 6th Adaptation Directive 91/422 EEC requires that when the unpressurized reservoir is filled, the spring-applied brakes must not release until the pressure in the service brake circuits is sufficiently high to achieve the effect required for the auxiliary brake when the service brake system (BBA) is actuated.


Directive 98/12/EC additionally requires, for vehicles first registered after March 2001, that actuated spring-applied brakes must not release until the pressure in the service brake circuits ensures at least the residual braking effect when the vehicle is loaded with the BBA. No special maintenance beyond the legally required inspections is required. The closing pressure (or stabilizing pressure) is tested with the engine stopped.


When a circuit is vented, the circuits that have not failed may only drop to the respective closing pressure. This test is to be repeated, after refilling the system, with a "simulated defect" in each of the other circuits.

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