Maximum pressure valves for diesel locomotives are used to adjust and maintain a reduced pressure in the brake cylinder, which improves braking efficiency. When the pressure rises above a certain level set by the degree of screw tension, the piston cuts off the air supply from the main air reservoir, and resumes only when the pressure drops to an acceptable value.
The 3MDA has an additional inlet which is connected to the brake cylinder outlet. If the brake cylinders are large enough, the design of 3MDA does not allow to fill them effectively with air, this process is very slow, because the pressure above the closing piston builds up faster than in the cylinder.
In this case, a better choice is the 3MDA, which has an outlet from the brake cylinder acting on the closing piston.
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