The article below was scanned, then assembled from one volume of the Science of Railways series. The binding is stiff and brittle so it was not possible to force the pages perfectly flat on the scanner.
Retaining Valves in General:
Engineers operating on lines with flat or average grades ... would want a normal release of the car brakes as they continued to run a train along the track. Releasing the brakes happened through the same process as that used to refill the cars' air reservoirs so the reservoirs would be ready for the next brake application. Normally, releasing and recharging brakes at the same time did not create a safety problem.
However, engineers descending very steep grades would need the ability to refill the cars' individual brake systems WITHOUT the train running away down the grade. The retainer valve on each car was designed to be 'turned up' before trains descended steep grades. While engineers could recharge their brake system at the 'normal' rate, the train brakes would release at a special 'slow' rate because of the retaining valve adjustment made to all of the cars before descending the grade. After the bottom of the grade was reached, the retainers were turned 'down' - back to their normal setting.
* * *
With the original retaining valve:
With the handle in the 'six o'clock' position, air exhausts from the car brake cylinders without impediment to atmosphere.
When the retainer handle is turned up to the 'three o'clock' position as it is seen on the diagram, the car brake cylinder exhaust must lift a weight off its needle valve seat, then the exhaust must thread its way through the small exhaust port marked 'd'.
... On the CPR's Big Hill, trainmen working a passenger train downhill were instructed to ensure this aperture was clear using a 1/16 inch wire.
from: The Science of Railways; 1891-1900; The World Railway Publishing Co.
... The Westinghouse High and Low Pressure Retaining Valve used the same handle positions for the same effect as the first retainer illustrated ...
However, for the circumstances when EXTRA car brake cylinder pressure was to be retained as the engineer recharged the brake system while in motion, an INTERMEDIATE handle position between 'turned down' and 'turned up' was created. This intermediate position was 'stronger' than the normal 'turned up' setting. The High Pressure setting forced the exhausting brake cylinder air to lift an ADDITIONAL weight.
As train air brake experience was gained, and as motive power and car weights increased, it was determined that this new setting was necessary to maintain control of heavy trains on steep grades.