Master Cylinder

I. Open vs. Closed

A. Open System:

Open systems include a reservoir and bladder that allow for fluid to be automatically removed or added to the hydraulic system during usage.  This bladder is at atmospheric pressure such that it can easily expand or contract.  Reservoirs are the overflow for fluid that has heated and expanded due to braking.  The ability for a bladder to change its volume within the system gives the excess fluid volume due to expansion a place to go without affecting the performance or feel of the brake.  On the flip side, as brake pads begin to wear in a hydraulic caliper, the caliper pistons will move out further in their bores to adjust for this wear.  The reservoir bladder automatically supplies the extra fluid needed to allow the pistons to pump out further.  Having an open system that is either under-filled (bladder under vacuum) or overfilled (bladder under pressure) can greatly affect the performance of the brake.

B. Closed System:

A closed system may have a reservoir of excess fluid, however there is typically no bladder to automatically adjust for the changes in volume due to heat or pad wear.  When the fluid heats up, the volume must be manually increased in order to prevent the brake from locking on.  Vice-versa, when pads wear, the system must be manually adjusted to compensate for the wear.

II. Internal Components

The internal design of a hydraulic master cylinder includes:

A. Bore

Internal cylinder that the master cylinder piston slides in to generate pressure.

B. Push Rod (Lever connection/linkage)

The push rod transfers the force put in at the lever to the MC Piston.

C. MC Piston

Typically captures the seals of the master cylinder and slides in the bore to push fluid and generate pressure.

D. Primary Seal

Primary seals are typically housed within the master cylinder piston and push the fluid toward the caliper and generate pressure.  Primary seals can be an o-ring or a cup style seal.  Cups allow for lower lever efforts and the ability for a brake to be “pumped” up by allowing fluid to flow from the reservoir, over the top of the cup into the system.

E. Secondary Seal

Rides on the back of the piston and keeps the volume in the reservoir sealed when the brake is actuated.

F. Return Spring

Returns the lever and piston when lever input is removed.  The return spring force sets the feel for the brake system during dead-stroke.

G. Reservoir & Bladder

The function of the reservoir and bladder is defined in the open system section.  Below are a couple key attributes of reservoirs and bladders.

1. Construction – Bladders are typically manufactured from thin walled rubber to allow them to deform via expansion or contraction.  Bladders need to be open to atmosphere or basically have brake fluid on one side, and then are exposed to the atmosphere on the other.  This allows the pressure or vacuum inside the system to change the bladder shape without fighting any external back pressures.
2. Bladder Design – Reservoir and bladder systems in bicycles need to be free of air.  If air is present in the reservoir, it could work its way into the system when a bike is oriented in a certain way during storage, maintenance or a crash.  For this reason it is important that the reservoir fluid be free of air.  There are two says to accomplish this:
   
• Knife Style Bladder – When a standard reservoir is filled with fluid, a knife bladder can be inserted, displacing the fluid and any air present to spill over the side of the reservoir.  This can guarantee there is no air in the reservoir, however it is a messy process.
• Hayes Style Hydraulic Reservoirs – Hayes MC reservoir bladders are designed to stay assembled within the MC during the fill process.  The internal shapes are designed to force air out the exit bleed port and guarantee there is no air present.  This is a much more reliable and clean way for filling the brake system.