Other valves

Other brake valves perform supporting functions such as preventing backflow, limiting pressure, selecting pressure and providing rapid exhaust. A fault can therefore cause various symptoms: brakes that release slowly, pressure loss while stationary or a parking brake that does not fully release. First establish the function of the brake valve in question in the vehicle’s system diagram. A leaking exhaust port does not automatically prove that this particular component is faulty; compressed air flowing back from a connected component can produce the same symptoms.

Depending on the brake system configuration, this category may include non-return valves, shuttle valves, quick-release valves, pressure-limiting valves and overflow valves. Non-return valves prevent backflow; shuttle valves generally select the higher of two control pressures. Quick-release valves shorten the exhaust path when the brakes are released. Pressure-limiting valves limit the outlet pressure, while overflow valves only supply a downstream circuit once a specified opening pressure has been reached.

In truck and bus air brake systems, non-return valves help maintain pressure in a connected circuit if the supply pressure is lost. Shuttle valves are used, among other things, to route two pneumatic control signals to a single outlet without interconnecting the control lines. Quick-release valves exhaust air close to the brake actuators, so it does not have to flow back through a long line when the brakes are released. Pressure-limiting valves keep the supply pressure to a connected brake component within the specified value; unlike pressure relief valves, they are not intended to vent excess system pressure. Separate overflow valves can ensure that an additional circuit is only supplied with air once sufficient pressure has built up in the upstream circuit.

Separate, purely pneumatically controlled relay valves may also fall within this category. They use a control pressure to regulate a larger flow of air locally to the brake actuators. This speeds up brake application and release where long pipe runs are involved. These are standalone relay valves, not relay functions integrated into ABS or EBS valves or an axle modulator.

On MAN TGL, TGS and TGX, Mercedes Atego, Actros and Arocs, and buses such as Lion’s City and Citaro, port configurations and pressure characteristics vary according to the brake system version. For trucks, buses and vans, therefore, always identify parts using the vehicle’s chassis number, together with the part number and the pneumatic or hydraulic circuit diagram. Visual comparison or selection based on the model name alone is never sufficient. MAN TGE and Mercedes Sprinter generally have hydraulic service brakes; pneumatic auxiliary brake valves are therefore not directly interchangeable with these systems. The brake configuration is more important here than the engine family.

On vans with a vacuum brake servo, the vacuum non-return valve serves a different purpose. This valve is fitted in the vacuum supply to the brake servo and helps retain the vacuum when the vacuum level on the supply side drops. Leakage can reduce the available brake assistance and increase the pedal force required. Check whether the valve can be replaced separately or forms part of the specified pipe assembly.

Compare the maximum operating pressure, opening and closing pressures, hysteresis, flow capacity, port assignments, threads and sealing method. Versions with or without backflow must also match. Aluminium housings, brass fittings and elastomer seals need to be checked for corrosion and compatibility with the operating medium. Use only the specified tolerances: an identical connection layout does not guarantee an identical control function.

Condensate causes corrosion and can block moving valve components in freezing conditions. Compressor oil in the compressed air can damage seals; dirt between the valve seat and seal causes internal leakage. Worn guides, hardened diaphragms and broken springs lead to incorrect switching pressures or delayed pressure release. Repeated failures therefore also call for inspection of the air dryer, air Compressor and reservoirs.

1. Secure the vehicle safely with wheel chocks and follow the workshop procedure for pressure testing. On electronically controlled systems, first read out the fault codes and measured values from the EBS (Electronic Braking System).
2. Connect calibrated pressure gauges or pressure transducers to the specified test points for supply, control and outlet pressure. Record the system pressure and test conditions; compare them with the vehicle specifications.
3. Apply the brakes in a controlled manner and record pressure build-up and release over time. Persistent outlet pressure after brake release may indicate a blocked exhaust or a sticking valve element.
4. Check for external leaks using a suitable leak detection fluid. Then measure the pressure loss over the specified test period. Locate internal leakage using the circuit diagram; do not isolate safety circuits arbitrarily.
5. Check the opening pressure, closing pressure and backflow behaviour according to the manufacturer’s procedure, using a suitable test bench if necessary. Only replace the valve once the supply lines and connected brake actuators have been ruled out as the cause.

Before removal, depressurise all affected circuits and secure the vehicle independently of the parking brake. Label the lines with their port numbers, keep open connections clean and check the line ends for damage. Fit new seals of the specified type, observe the tightening torques and prevent sealant from entering the air passages. Do not alter factory settings to make an incorrect version work.

After installation, check for leaks, verify the switching pressures and ensure that the brakes release fully. Then carry out the specified brake performance test, including a check on a brake tester where required. This prevents residual pressure, contamination in the lines or an unresolved air treatment fault from causing further downtime.