. Air brake text for engineers and firemen; . equalizeit will be a leaky rotary, but if the pressure does not equalize thetrouble will be caused by a leaky excess pressure valve. 44 ENGINEERS BRAKE VALVES. Gauge Indications. When the brake valve handle is mid-way between the service and full emergency positions the blackhand may show main reservoir pressure, although it is knownby the position of the valve that there is no air in the brake indication is due to the construction of the valve. When itis in this position, port / of the rotary stands over port g in therotary seat, which l
. Air brake text for engineers and firemen; . equalizeit will be a leaky rotary, but if the pressure does not equalize thetrouble will be caused by a leaky excess pressure valve. 44 ENGINEERS BRAKE VALVES. Gauge Indications. When the brake valve handle is mid-way between the service and full emergency positions the blackhand may show main reservoir pressure, although it is knownby the position of the valve that there is no air in the brake indication is due to the construction of the valve. When itis in this position, port / of the rotary stands over port g in therotary seat, which leads to chamber D. Chamber D is chargedto main reservoir pressure and the black hand registers chamberD pressure. EQUALIZING RESERVOIR. The small equalizing reservoir or brake valve reservoir, com-monly called the little drum (Plate 17), is usually placed underthe cab footboard. A pipe leads from this small drum to theengineers valve which is connected at T, as shown on Plate air can then pass through port ^ and chamber D. The equal-. PL ATE 17, izing reservoir serves to increase the capacity of chamber D,without an enlargement of the engineers brake valve. ChamberD being of a small capacity, it would be impossible to make agradual service application of the brakes without the increasedvolume furnished by the little drum. ENGINEERS BRAKE VALVES. 45 The black hand of the air gauge is connected at the brakevalve, and it therefore indicates chamber D and brake pipe pres-sures. The equalizing piston 17 separates chamber D from thebrake pipe pressure. The pressure in chamber D acts in holdingthe equalizing piston dov^n, and brake pipe pressure tends to forceit upward. Purpose of the Small Drum. The purpose of the smalldrum is to increase the volume of air on top of the equalizingpiston in the engineers brake valve. The air in the small cavityover the equalizing piston is sufficient to hold the piston to itsseat, but there is not a sufficient volume to drav^ from whenmaking a
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