In spite of the most modern test methods, hopefully, when repairing, not all the feeling for the vehicle itself has been lost. The read-out of the error-memory does not always lead to success. Sometimes it is worthwhile to trust your hands and other senses. Imagine if, eg., a truck cannot be moved at all. You operate the brake pedal, but because the vehicle is not moving, the effect can't be checked.In this case, it's an advantage if the various reservoirs still have - or again have - drainage valves which can be easily opened. Simply try them out, one after the other, you may then notice that the reservoir for the hand-brake has built up insufficient pressure. Of course, it won't always be as simple as that to find the problem. The next step is, to take the measurement readings. This is partly possible by electronically analysing measurement-value-blocks. Should the computer not deliver the desired values, the manometer, shown above in picture 2, should be applied. In most compressed-air systems the right-hand one is sufficient. The one on the left is, more than likely, for the combined hydraulic-pneumatic systems. For a number of measurements in the purely compressed-air area, we need two. In that way, eg., the order in which the the reservoirs are filled, or the pressure development between the front- and rear axle with increasing pedal-pressure, can be checked. Now, we want to find the respective connections on the rear-axle.
Actually, it's really very simple. What we have is a pneumatic-sprung rear axle with a combi-brake-cylinder on the left and on the right. Each has two pipes. The question of which connection belongs to the hand-brake and which to the brake-pedal can be clarified in a number of ways. If you look at the figure, you can see that on the left, there is only one- and on the right-, both pipes have a control connection. Due to the fact that the hand-brake only needs one, the left hand side control-connection belongs to the brake-pedal. If you now follow the other pipe, you come to the relay valve. Thus, both of the right hand side control-connections are also clarified. You can, of course, also start with the only left hand side control-connection. This pipe leads to the ABS-valve, to which there is a corresponding on the other side. Thus, both of the right hand pipes are clarified. If this is still not enough for you, then carefully go one step closer and have a look at the description on the supply points directly on the respective combi-cylinder. Whereby the brake-pedal is marked with '11', and the hand-brake with '12'.