1. Clutch Master Cylinder Anatomy
This is the master cylinder in it's resting state. The hydraulic fluid can move freely in and out of the reservoir which is why the hydraulic clutch can self adjust to wearing out disc and components unlike a mechanical clutch. It also buffers the thermal expansion and contraction of the clutch fluid by sending the extra volume to/from the reservoir.
If you take a look at the relief valve you can see the relief valve spring pressing on the valve to close the relief valve shut but in it's resting state the stopper's position pushes the valve open so fluid can freely travel in and out. The stopper's position where it can push open the valve when the clutch pedal is released is critical for the proper operation of the 2G clutch master cylinder.
2. Pedal Free Play
Let's see what happens when the clutch pedal is initially depressed before you feel any real resistance.
Pedal free play is the amount of pedal travel before you can feel any resistance. It's when the clutch pedal is depressed just enough to close the relief valve and not pressurize the lines and/or manipulate the slave cylinder. In reality it's almost impossible to distinguish the exact ending point unless you have someone watching the slave cylinder rod until it moves.
3. Disengagement - Pressurizing Zone
Depressing the clutch pedal beyond the pedal free play is where you are actually building pressure to manipulate the slave cylinder rod to disengage the clutch.
Since the relief valve is now closed shut you're building hydraulic pressure as you further depress the clutch pedal. This pressure buildup also aids in tightly closing the relief valve just like the hydraulic lash adjusters. The amount of travel measured at the pedal when your clutch releases is your clutch disengagement point.
The clutch fluid expands as it heats up during normal use and when you release the clutch the stopper push opens the relief valve and the expanded fluid can flow into the reservoir relieving any pressure in the lines.
4. The Clutch Pump Up!
"Pump up" is when your disengagement point gets higher as you drive the car, not meaning highway cruise but normal city driving when you're constantly shifting. This happens when you mindlessly thread out the master cylinder rod to gain more pedal travel when you have a heavy clutch like the ACT, which tends to disengage close to the floor. If you read the above sections you already know why this happens.
When you thread out the rod, it's pushed into the master cylinder. Pedal free play is the amount the relief valve is still open. If you go past this distance, as you can see in the diagram above, the piston assembly moves away from the stopper and the relief valve is now always closed preventing any fluid movement to/from the reservoir.
Keep in mind that during normal use the hydraulic fluid expands due to heat. This extra volume needs to go to the reservoir whenever you let off the clutch but since the relief valve is closed the slave rod extends and applies pressure to the clutch just as if you were to foot the pedal. More you use the clutch, more it expands, so you notice your disengagement point slowly rising. It also means you are constantly riding the clutch and bad things happen to your release bearing and/or clutch. This all goes away in the morning or after a long park since the fluid cold contracts to it's initial volume. In some instances you may find the pedal drop to the floor when the weather gets much colder in the morning. This isn't crankwalk, the assist spring is strong enough to drop the pedal to the floor when you lack any volume in the clutch line. Pump it a few times and it goes right back up.
5. Clevis Pin Free Play
This is the miniscule amount of free play movement at the clevis pin when you depress the clutch pedal without any movement of the rod itself. If you have clevis pin free play, then it means the master cylinder rod isn't pushing against the pedal or the piston. So you know the master cylinder piston has full stroke and is in it's correct position relative to the stopper to keep the relief valve open whenever you release the clutch pedal.
6. Adjusting your Clutch Pedal
Adjusting the clutch pedal is easier than you think. First adjust the pedal stop bolt (or the cruise control switch) on the top so that the pedal height(1) is 175-180mm from the firewall. Pedal height is measured from the firewall to the top of the pedal pad. After setting the pedal height you need adjust the rod so that you have 1-3mm clevis pin free play(2). Clevis pin free play is also measured at the pedal. On a stock setup when you adjust the pedal height and clevis pin free play, then the pedal free play(3) and disengagement height(4) will naturally be at factory spec.
7. How to raise your Disengagement Point
The proper way to raise the disengagement point is to thread out the top bolt so the pedal rest as high as possible and adjust the clevis pin free play to factory spec. If that wasn't enough then you need to sacrifice pedal free play. Thread out the rod to make it longer so it extends into the master cylinder. You loose clevis pin free play and pedal free play but this will make the master cylinder go into pressurizing zone earlier. After adjusting the rod you need to check whether or not you're in pump up mode. Pump the clutch a few times and go under the hood and try pushing in the slave rod. It should go in with a low to moderate resistance. If it doesn't then you're in pump up mode and will need to shorten the master cylinder rod.