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The brake Master Cylinder

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This series is divided into seven sections:
Part 1 - Why it goes bad and what it feels like (and why your pedal goes to the floor)
Part 2 - Removal and replacement
Part 3 - How to bench-bleed the new Master Cylinder
Part 4 - What the Master Cylinder looks like inside
Part 5 - How the Master Cylinder works, and why the pushrod needs to be set correctly
Part 6 - How to check the pushrod freeplay (a link to the bottom of Part 2)
Part 7 - How to adjust the pushrod freeplay
Note: All pictures can be clicked on for LARGER versions!

PART 5: How the Master Cylinder works, and why the pushrod needs to be set correctly

Below is a schematic showing the parts of the Master Cylinder assembly. The two lines at the bottom go to the proportioning valve, then to the wheels.

Since this is a dual-circuit system (like all US-legal brakes since 1968), there are two separate paths for the fluid to follow, and two separate pressure-generating chambers for the lines. Most (if not all) Hondas have "dual-diagonal" systems, meaning the right-rear and left-front wheels are on one circuit, and the left-rear and right-front ones are on the other. Some makers, such as Toyota, tend to use a front/rear split. This makes a difference in how you bleed the system.

The reservoir is common to both circuits, splitting only where the vertical wall is, in the MC body.

The Master Cylinder, at rest, pedal released:

The parts of the Master Cylinder

Foot pedal "free play" location:
Detail of freeplay area
And this is where the foot pedal's "free play" comes from. It is absolutely necessary that there be some amount of clearance, otherwise the seals may be pushed forwards enough to prevent the intake/return ports from ever opening up again to allow excess pressure to vent to the reservoir.

The factory manual for my car specifies 1mm to 5mm clearance.

The intake/return ports are less than a millimeter in diameter, and the seal lips sit immediately behind them, so it doesn't take much of a misadjustment to close off the ports, even partially.

Too little clearance here may cause the brakes to lock on slowly as you drive. Too much clearance will make your pedal feel soft, with excessive travel. (More info below, in red)

Notice the blue plug at the back? It's actually black steel, and has two O-rings: One for the MC body and one for the plunger. It's entirely independent of the hydraulic seals that push the fluid around, and that's why a leaky Master Cylinder won't drip anywhere that you can see. Fluid squirting past the seals simply goes back up the equalization ports.

Nothing you do at the pedal affects clearances inside the MC body until the pushrod starts to push the seal assemblies forward.
(proportioning valve not shown in graphic above)

Within first 3/8" of pedal movement:
Pressure builds in 1st circuit
Everything starts to happen within the first 3/8" of pedal movement. That translates to about 2mm at the pushrod.

The arrows indicate fluid direction and also indicate where it's moving.

Notice that as the second circuit rear seal moves forwards, fluid must pass down its equalization port to prevent suction from forming behind the rear seal.

The first circuit's return spring is quite a bit lighter than the second circuit's return spring. This means that initial fluid pressure does not begin to build in the second circuit until it has started to build in the first.
This is how pressure equalization is effected between the circuits.

Effect at the wheels is minimal right now.

Once pressure in the first circuit is present and is high enough (which is almost immediate), the second circuit's spring will begin to compress...

Still within first 3/8" of pedal movement:
Parts when braking effect is seen in both circuits ...as we see here
Pressure in the first circuit is now high enough to overcome the spring in the second circuit, and the pushrod has begun to pressurize the fluid in the second circuit.

At ths point, braking effect can be felt at the wheels. The pedal still has not traveled much more than 3/8" to 1/2".

Of course, if the system is in poor shape, has air, or is misadjusted, the pedal will need to  travel further before pressure is present.

I haven't shown arrows at the equalization port for the first circuit. There is fluid displacement here, but it's tiny, mostly having to do with seal distortion. The solid spacer itself does not compress, unlike the parts with springs around them.

At maximum braking effort (about 1.5" of pedal travel):
Parts near maximum foot pressure
Full braking effect is now available in both circuits. The pistons and seals will continue to move forwards, squeezing the fluid and increasing pressure as long as you keep pressing harder on the pedal.

A system in good shape will have a pedal that moves about 1.5" to achieve maximum braking effort. That's about 1/2" at the pushrod. In this graphic, the seals are shown approximately at their point of maximum travel. In other words, this driver is pressing hard on the pedal.

See the nose of the solid spacer (yellow)? It still has some room left to travel before it hits the front of the Master Cylinder body. That room gets used when you pedal-pump bleed the system..

The surface immediately ahead of where the seals are in this graphic is the area that the seals normally never travel on. This is the area that collects gunk if the system is neglected. This is the area that tears up the seals if the pedal-pump bleeding method is used on a car that has had its brake fluid left in too long (more than 5 years).

If you change your fluid annually with the pedal-pump method, this untravelled area will get a regular sweeping to keep it free of gunk.

The pedal has been released:
Pedal released, pressure dissipated
When the pedal is released all the way, the return springs push everything back home again. As the seals return home, they uncover the intake/return ports.

When you brake, the fluid heats up, both from the act of compression, and from heat generated by the friction surfaces. This causes the fluid to expand, so the volume of fluid after braking is larger than it was before braking. The excess volume is vented through the intake/return ports when they reopen, allowing the wheel pistons to move back home and release pressure on the discs/drums.

Why the pushrod needs to be adjusted
If the pushrod is misadjusted too tightly, and never allows the seals to move rearwards enough to uncover the return ports properly, your brakes will slowly lock on more each time you brake, eventually bringing the car to a stop.

Allowing the fluid to cool (and thus contract) will release the brakes, so you can continue to drive until they lock up again.

Adjustment of the pushrod is critical!