Air cylinder

Engine brake pneumatic cylinders convert air pressure into mechanical movement of the exhaust brake butterfly valve. During braking, this valve deliberately increases exhaust back pressure, causing the engine to provide braking effort. A leaking or sticking actuator reduces braking performance or prevents the valve from opening fully during acceleration. These components form part of the exhaust brake actuation system and are not interchangeable with service brake cylinders.

Always identify pneumatic cylinders using the vehicle's chassis number, together with the original part number and the engine brake version fitted. Visual comparison or selection based solely on the model name is never sufficient. On MAN vehicles with, for example, D08 or D28 engines and Mercedes vehicles with OM 904, OM 906 or OM 457 engines, the actuation system varies according to the year of manufacture and exhaust system. An exhaust brake cylinder is also not an actuator for a compression-release brake, even when both braking principles are combined.

On MAN and Mercedes trucks, buses and vans, first check that a pneumatically operated exhaust brake is actually fitted. This is not standard equipment, particularly on vans. Distinguish between single-acting pneumatic cylinders with a return spring and any double-acting versions; their connections and control arrangements are not interchangeable.

Compare the cylinder bore, stroke length, permissible operating pressure, spring force, connection thread and mounting centre distance. The available actuating force is determined by pressure multiplied by the effective piston area, minus spring force and friction losses. Rod length, clevis, pin fit and mounting orientation must also match. Corrosion protection of the steel piston rod, housing material and the temperature resistance of the elastomer seals are critical near hot exhaust components. Use only the vehicle-specific pressure values and adjustment tolerances.

Typical symptoms include weak engine braking, delayed engagement, audible air leakage or loss of power after disengagement. Worn piston seals cause internal leakage; corrosion on the rod damages the rod seal. A broken return spring, worn pivot pins or a carbon-fouled valve shaft can cause similar symptoms. Therefore, do not automatically replace the cylinder if the exhaust brake does not move.

1. Make safe: secure the vehicle, allow the exhaust to cool and depressurise the relevant pneumatic circuit before removal. Be aware of stored spring energy; do not open a sealed spring housing.
2. Check the control system: read out any available fault codes and activation conditions. If supported, carry out an actuator test and check the electrical supply to the electropneumatic control valve against the wiring diagram.
3. Measure pressure: connect a suitable pressure gauge at the cylinder air inlet. Compare pressure build-up during engagement and pressure release during disengagement with the manufacturer's specifications. Check connections with leak detection spray. Residual pressure may indicate a blocked exhaust port or a faulty control valve.
4. Isolate the mechanical components: disconnect the operating rod only when the system is depressurised. Check that the valve shaft moves freely and measure the cylinder stroke with a vernier calliper during a controlled test at the specified pressure. Assess return movement, play and any pressure drop in accordance with the test procedure.

Replace damaged pneumatic cylinders and, at the same time, repair leaking air lines, worn linkages or a sticking exhaust brake butterfly valve. Adjust the operating rod according to the manufacturer's instructions: an incorrect length can prevent full opening or force the actuator against its end stop. Use the specified locking devices and tightening torques; avoid side loading the piston rod.

Refit heat shields and, if moisture or corrosion is present, also check the air dryer and condensate drain. After installation, run several operating cycles to check for leaks, full stroke and free return movement. Then verify under safe test conditions that the engine brake both engages and releases fully. This prevents the new component from suffering the same consequential damage.