Brake pressure regulator

A brake force control valve regulates the pneumatic brake pressure to the rear axle(s) according to vehicle load. On an unladen truck, bus or van, the valve limits pressure to the rear brake chambers to prevent premature wheel lock-up. At higher axle loads, it allows more compressed air through so that the available grip and braking capacity are used correctly. This valve is also referred to as a load-sensing brake force control valve or ALB valve (Automatic Load-Dependent Limitation) and is particularly relevant for vehicles with mechanical suspension, leaf springs or an axle-mounted operating rod.

Brake force control valves are found on older and specific versions of the MAN F8, F90, F2000, M2000, L2000, TGL, TGM, TGA, TGS and TGX, as well as the Mercedes LN, Ecoliner, SK, Atego, Axor, Actros and Arocs. They are also applicable to certain Mercedes Sprinter, Vito, Vario and MAN TGE versions, where a hydraulic or pneumatic load-dependent control system may be fitted depending on the specification. In city and coach buses, such as the MAN Lion’s City, Lion’s Coach, Mercedes Citaro, Intouro, Tourismo and Neoplan Cityliner or Tourliner, the function may be integrated into an electronic EBS (Electronic Braking System) or ECAS (Electronically Controlled Air Suspension) system rather than a separate mechanical valve.

A conventional pneumatic control valve is connected to the chassis rail and rear axle via a lever, pull rod, spring mechanism or linkage assembly. The position of the axle relative to the chassis determines the internal valve position. The connection configuration, nominal supply pressure, flow capacity, control characteristic and lever geometry are among the determining factors. A valve with an incorrect pressure curve may cause the rear axle to lock up when driving unladen, or provide insufficient brake pressure when loaded. Selection based solely on model name, year of manufacture or visual comparison is therefore never sufficient: the correct brake force control valve must always be identified using the vehicle chassis number.

Internal corrosion caused by moisture in the compressed air, contamination from compressor oil, damaged diaphragms and worn O-rings are common causes of faults. A seized lever, bent pull rod, worn ball joint or incorrectly adjusted operating rod can also alter the pressure control. Typical symptoms include rear wheel lock-up when driving lightly loaded, an excessively long braking distance under full load, uneven brake lining or brake pad wear, abnormal readings on a brake test bench and continuous air loss through the valve exhaust port.

First check whether the air tanks build up the specified system pressure and whether the air dryer is functioning correctly; water or oil mist in the circuit will cause renewed damage to a new control valve. Then inspect all compressed air lines, fittings and connections using leak detection spray. Next, use calibrated pressure gauges to measure the supply pressure and outlet brake pressure under several load conditions or simulated lever positions. Compare the measured pressure ratio with the vehicle-specific test values specified by the manufacturer. At the same time, check the free movement, mounting points and basic adjustment of the operating rod. On vehicles with ABS (Anti-lock Braking System) or EBS, it must also be ruled out that a fault in wheel speed sensors, modulators or wiring is affecting brake pressure distribution.

Replace a brake force control valve when pressure values are outside tolerance, the housing shows corrosion damage, the control lever seizes or internal leakage can be demonstrated. Only dismantle it after the pneumatic brake circuit has been fully depressurised. Protect open lines from dirt ingress and use only lines, sealing rings and couplings with the correct diameter, thread and pressure rating. Do not force old lines into a new connection; stress in the compressed air line can cause leakage or an incorrect valve position.

After installation, adjust the pull rod according to the specified ride height and axle-to-chassis distance. Always carry out a leak test, pressure comparison and brake test bench inspection before putting the vehicle back into service. A readily available suitable brake valve and technical advice can minimise downtime, but final verification of brake balance remains essential for safe and compliant operation.