Brake cylinder

A brake cylinder converts the regulated air pressure from the pneumatic brake circuit into a linear mechanical force for the service brake. For MAN and Mercedes trucks, buses and vans, this category covers single-chamber diaphragm brake cylinders, also known as service brake cylinders. The force is transmitted via a push rod, clevis and slack adjuster to the brake camshaft of drum brakes; in certain applications, the actuator operates a pneumatic brake caliper. Correct selection is essential: a brake cylinder must always be identified using the vehicle chassis number. Visual comparison or selection based solely on the model name is never sufficient.

Brake cylinders differ in diaphragm diameter, effective piston area, stroke length, mounting method, port connection and push rod length. Dimensions are often specified in area classes, for example type 16, 20, 24 or 30. A larger effective area provides greater actuating force at the same operating pressure, but must correspond to the axle load, brake calculation and fitted slack adjuster.

Applications can be found on MAN F90, F2000, TGL, TGM, TGA, TGS, TGX and TG3, as well as on Lion’s City, Lion’s Coach and Neoplan buses. For Mercedes, these include LN, SK, Atego, Axor, Actros, Arocs, Econic, Citaro, Intouro and Tourismo. Pneumatic brake cylinders are mainly used on heavy vehicles with engine families such as MAN D08, D20, D26 and D38 or Mercedes OM 906, OM 457, OM 471 and OM 473. Depending on the version, Sprinter, Vito and TGE models may feature hydraulic wheel cylinders, brake caliper pistons or hydraulic master brake cylinders; these are not technically interchangeable with air brake actuators.

Typical faults include a torn diaphragm, internal corrosion of the cylinder housing, a seized push rod, leaking compressed air connections or a weakened return spring. Recognisable symptoms include air loss during braking, delayed release of a wheel brake, uneven braking on one axle or an EBS fault message (Electronic Braking System) due to an abnormal brake response.

First check the system pressure on the instrument panel, then use a calibrated pressure gauge to measure the pressure at the brake cylinder inlet. Compare the pressure build-up and release on the left and right sides of the same axle. A noticeable pressure difference is more likely to indicate a foot brake valve, relay valve, ABS valve (Anti-lock Braking System), line or coupling than the actuator itself. Pressurise the connections and diaphragm edge and use leak detection spray; bubble formation confirms an external leak. Then measure the push rod stroke under full brake application and compare it with the permissible stroke for the cylinder type. Excessive stroke may also be caused by incorrectly adjusted slack adjusters, worn brake linings, a defective brake camshaft or seized brake components.

Replace brake cylinders in the event of leakage, mechanical damage or when the permissible push rod stroke is exceeded. Only fit a version with identical stroke, force class, connection thread, clevis size and mounting fitment. After installation, check the free movement of the push rod and slack adjuster, perform a leak test and test the service brake. Replace damaged air hoses, couplings and mounting hardware at the same time to prevent repeated downtime. Available stock and technical support at Braem help ensure brake repairs are carried out safely and efficiently.