This is the engine compartment of the famous Mercedes 190 SL (1955 - 63). And despite its huge brake booster (lower half right), its brake system is operated by a single-circuit master brake cylinder. The 220 SE Coupé built from 1961 onward also has six cylinders with fuel injection and crumple zones for passive safety, but despite disk brakes it has only one brake circuit at the front (arrow).

The single-circuit master cylinder does not differ in terms of repair or reconditioning from the one with two pistons. In principle, the function and sequence of the components to be assembled must be absolutely clear. This is by no means intended to be a repair manual, but it does recommend the use of dial gauges and honing equipment.

The latter should not be imagined too elaborately. It can, for example, consist of two small honing stones that are pressed against the wall of a brake cylinder by springs and set in rotation by a hand drill. Of course, the dilatation, which may be associated with this smoothing, must be checked, may be with an internal dial gauge.

Since rubber parts are difficult to repair, a repair kit is recommended. If this is no longer available, e.g. for vintage cars, it may be possible to replace it with a more common one, even with the approval of the main inspection. However, you should test it with simulated failures of one brake circuit and the other.

In general, faults in the brake system are relatively easy to detect, leaks for example, by pressing the brake pedal as hard as possible with a brake booster while the engine is running. Also multiple actuation, the so-called pumping, can show faults if the pedal travel changes significantly. Of course the pedal travel itself is also a criterion.

As already mentioned, warranty is not given, but when we describe such repairs, we also mention the basic rules, e.g. never overhaul the wheel brake cylinders or linings of only one side. By the way, with drum brake cylinders the short loupe of the protective rubber is an important indicator for leaking brake fluid.

In addition to the already mentioned initial position of the two pistons, the position of the pedal to the first piston is particularly important. Normally, approx. 1 mm play of the push rod is required here when the pedal is not actuated. Attention should also be paid to connected bottom valves on drum brakes and special bottom valves on disk brakes. These valves have even penetrated into the ABS era, e.g. for both brake circuits in versions without ABS.

Although usually in or near the master cylinder, bottom valves can be installed almost anywhere. They should not be interchanged under any circumstances, because the valves for disk brakes basically only contain throttles (special bottom valves), which promote a slower backflow of the brake fluid, while the bottom valve for drum brakes maintains a residual pressure of 0.4 to 0.8 bar.

Perhaps not only for classic cars is a regular inspection independent of inspection intervals. Quite convenient is the regular check of the expansion tank, if necessary even including markings (picture above). However, when the fluid level changes, the wear of the pads must be taken into account.