Brake accelerator for a fluid pressure brake system

A brake accelerator for a fluid pressure brake system having a brake line and an auxiliary chamber with a throttle connection to said brake line, and comprising first means for defining a first chamber having a first piston movable therein between first and second positions, said first piston defining on one side thereof a first sub-chamber connected to the brake line and on the other side thereof a second sub-chamber connected to said auxiliary chamber, said first piston being in the first position when a pressure equilibrium exists in the first and second sub-chambers during release position of the brakes and being in the second position when the pressure in said second sub-chamber is greater than in said first sub-chamber during a service braking operation, a control piston movable in a second chamber between first and second positions, said control piston defining on one side thereof a third sub-chamber and on the other side thereof a fourth sub-chamber, first passage means between said third sub-chamber and the first sub-chamber, a first valve in said first passage means operable between positions to open and close said first passage means with respect to said first sub-chamber, a second valve connecting said first passage means to said third sub-chamber when said second valve is in an open position, means for defining a first control chamber communicating with said first passage means, a third valve in said first passage means between said first valve and said first control chamber and movable between open and closed positions, second passage means in connection with said first passage means for drawing off pressure fluid from said first control chamber through said first passage means and said third sub-chamber to the atmosphere, third passage means communicating with said second sub-chamber, a fourth valve in said third passage means and movable between open and closed positions, second means connected to said third passage means for venting pressure fluid from said second sub-chamber when said fourth valve is open, third means for opening said first valve when said first piston is in its second position, fourth means for opening said third valve and for closing said second and fourth valves when said control piston is in its first position, said fourth means closing said third valve and opening said second and fourth valves when said control piston is in its second position, pressure fluid entering said first control chamber from the brake line upon opening of said first valve when a decrease in pressure occures in said brake line to increase pressure in said first control chamber such that the increased pressure acts upon said control piston in conjunction with said fourth means to open said second and fourth valves and to close said third valve whereby pressure fluid is drawn off to the atomosphere from said first control chamber and said second sub-chamber, said control piston in conjunction with said fourth means closing said second and fourth valves and opening said third valve when the pressure in said first control chamber drops below a predetermined valve so that a limited quantity of pressure fluid is drawn from said brake line and a predetermined pressure relationship is maintained on both sides of said first piston during a service braking operation.