Water Brakes-How did they work?

In June 1903, the D&RG and B&M [for couplers] petitioned the ICC for an extention to the Safety Appliance Act of 3/4/1893, ammended 3/4/1903 that was due to require driver-wheel brake shoes by Sept. 1, 1903. The D&RG claimed they did not have time to apply all the rigging on the narrow gauge engines and that they were already equiped with water brakes which served the purpose.
The courts extended the deadline from Sept. 1, 1903 to October 15, 1903 until a hearing was held. After the hearing, the "D&RG and the C&S were given until July 1, 1904 in which to apply to Congress for legistration to permit the use of Narrow Gage locomotives, without driver-brakes, on the understanding that a water brake is to be used." PRR