The master cylinder volume is to displace the area of the of the pistons or wheel cylinders.A proportioning valve controls the pressure.A stock valve and a race valve do the same thing they open and close the passing orfice which controls the pressure,ie: putting your finger on a garden hose to increase the pressure.Small race calipers with small pucks can use a small MC(less area,less volume)large street calipers with large or multiple pucks need larger MC bores and resovires. By adjusting the size of the passing orfice(making it smaller) increases the pressure,usually in to increase and out to decrease on the race valves