Master Cylinders

The master cylinder is the heart of the brake system. Actuated by surge or pump, its piston provides the force and the movement required to apply the brakes. When the pedal is released, an internal return spring returns the piston to its resting position.
 
Initially the piston moves forward and fluid volume is displaced, taking up all clearances in the system. This fluid movement actuates the caliper pistons which extend and bring the brake pads into contact with the rotors. Because the fluid is incompressible, once the pads are in contact with the rotor, fluid movement stops and pressure rises.
 
The critical specs of a master cylinder are its bore (diameter of the piston) and stroke (how far the piston can travel – and therefore how much fluid it can displace when applied). 

Reservoir Size

Disc brake reservoirs are larger than those for drum brakes. You will often see two reasons given for this:

1.    First, because the pistons in a disc brake caliper are MUCH larger than the tiny pistons in a drum brake wheel cylinder, disc brakes require more fluid volume to be displaced than drum brakes – requiring a larger reserve of fluid for operation.

2.  Secondly, as disc brake pads wear, disc brake calipers are self adjusting. That is, the calipers only retract the piston just enough to prevent pad-to-rotor contact. Now, imagine you start with disc brake pads with ½” thick linings and you have a 4” diameter caliper piston. Every time you apply the brakes and the pads wear a little bit, the caliper retracts just a tiny bit less. By the time the pads wear to 25%, or 1/8” thick, the piston at rest will be .75” further out in its stroke than it was when the pads were new. That .75” behind the piston must be taken up by additional fluid – and in the case of a 4” diameter piston, the additional volume required is given by (pi [d/2] ^2 * 0.75) or about 8 cubic inches. Multiplied by two calipers (one for each wheel) and that’s 16 cubic inches of extra fluid reserve required to compensate for pad wear. That’s much higher than the amount required to compensate for drum brakes shoe wear. Therefore, disc brake reservoirs are larger than for drums because OEM designers must design a reservoir for disc brakes large enough that the brakes will still function even if Joe Public doesn’t check the fluid or add a drop between new pads and completely worn out pads.  

Piston Size

It is true that disc brakes require both more pressure and more movement (volume) to operate than drum brakes.

Built-in Valve

This one can be a deal-killer. If the MC in question was designed for drum brakes and has a built-in residual pressure valve, it will not be suitable for disc/disc brakes. See section below on valve for description of residual pressure valves. For now, the point is, know the MC in question and whether or not it has built-in valve. If it does, you would have to either modify it by removing the residual pressure valve to make is suitable for use with disc brakes, or choose a different master cylinder.

Summary

The differences between disc and drum master cylinders are as follows:

1.    Disc brake MC’s normally have a longer stroke and larger reservoir than those for drums

2.      Drum brake MC’s have a built-in residual pressure valves.

Valve

Residual Pressure Valve

A residual pressure valve is a simple, one-way, spring-loaded valve installed either in the master cylinder.  They operate by keeping a pre-determined amount of pressure in the brake lines, even with the brakes released.  The internal spring determines the amount of residual pressure kept in the brake lines – normally 10 PSI. Here the use of a residual pressure valve:

10 PSI:  Drum brakes only. Because drum brakes don’t use calipers and are therefore not self-adjusting there are springs installed to retract the brake shoes away from the drum. A 10 PSI residual pressure valve is used in drum brakes to keep a little pressure in the lines to balance the return-spring force so that the shoes are maintained in close proximity to the drums. Without the residual pressure valve, the return springs would retract the shoes so far from the drums that excessive pedal travel would be required before the brakes are applied.