Device and method for detecting leaks in brake cylinder circuits

The method involves supplying brake fluid to the brake circuits (1) through a main air container pipe (2), and controlling the pressure (c) in a brake cylinder circuit (7) by a relay valve (6). A variable related to the brake fluid is measured and compared with the one or multiple reference values. The presence of the leakage in the brake cylinder circuit of the brake circuit or leak-free is indicated depending on the comparison result. Independent claims are also included for the following: (1) a monitoring device for a brake circuit of a train braking system; and (2) a train braking system with a supply.