Air tank

Air reservoirs are pressure vessels that store compressed air for the pneumatic braking system and, depending on the vehicle specification, for auxiliary consumers such as air suspension, clutch, doors or trailer control. In MAN TGL, TGM, TGA, TGS, TGX and TG3 trucks, Mercedes Atego, Axor, Actros, Arocs and Econic vehicles, as well as buses such as MAN Lion’s City, Mercedes Citaro and Neoplan coaches, these reservoirs form a safety-critical part of the compressed air supply. MAN TGE and Mercedes Sprinter versions with compressed air systems may also use specific air reservoirs.

An air reservoir should never be selected solely based on the model name, year of manufacture or visual comparison. Always identify the correct part using the vehicle chassis number. The exact specification is determined, among other factors, by the number of separate brake circuits, the position on the chassis or frame, the available installation space, the connection to the air dryer and the presence of a trailer circuit or air suspension.

Important selection parameters include the net volume in litres, the permitted operating pressure, the pressure vessel classification, wall thickness, material and connection configuration. Steel air reservoirs are robust and commonly used on heavy-duty trucks, but require intact corrosion protection. Aluminium pressure vessels reduce weight and are less susceptible to surface rust, but may be damaged by stone impact, galvanic corrosion around mountings or deformation of threaded ports. Also check the diameter and thread pitch of connections for brake lines, condensate drain valves, pressure switches and any sensors for the ECU (Electronic Control Unit).

The most common issues are external corrosion, internal rust formation caused by moisture, leaking connections, damaged condensate drains and cracking around weld seams, mounting brackets or threaded connections. A vehicle that builds up air pressure slowly, regularly loses air while stationary or displays a low brake pressure warning does not automatically have a faulty pressure vessel. A worn air compressor, a saturated air dryer cartridge, leaking brake lines or defective multi-circuit protection valves can cause the same fault symptoms.

First bring the system up to nominal system pressure and check the pressure build-up time according to the vehicle specification. Then switch off the engine, release the brake pedal and observe the pressure drop per circuit using diagnostic equipment or pressure gauges. Locate leaks using leak detection spray on weld seams, connection nipples, drain valve, blanking plugs and the transition between the line and fitting. Bubble formation indicates an external leak. Then drain each reservoir into a collection container. Rust-coloured water, oily condensate or excessive moisture respectively indicate internal corrosion, possible compressor oil carryover or insufficient drying.

Visually inspect the pressure vessel, paying particular attention to pitting corrosion beneath retaining straps and plastic protective covers. If in doubt, measure the wall thickness using ultrasonic thickness measurement; localised material thinning is more dangerous than uniform surface rust. A dented, cracked or heavily corroded reservoir must not be welded, filled or locally repaired: replacement is the only safe solution.

When fitting a replacement, also replace affected retaining straps, rubber isolators, sealing rings and corroded pipe fittings. Clean the line ends before installation to prevent rust particles from entering control valves, ABS (Anti-lock Braking System) modulators or EBS (Electronic Braking System) components. Use only suitable compressed air lines and fittings; incorrect thread engagement or excessive sealing paste can block passages or cause stress in the connection.

After installation, carry out a leak-tightness check and functional test of all compressed air circuits. Regular draining of condensate water and timely replacement of the air dryer cartridge significantly reduce internal corrosion. A readily available compatible air reservoir and technically correct identification by chassis number help minimise downtime for trucks, buses and vans.