Commodore Brake Master Cylinder Movement

What Is It ?

If you read the LS1 Forums at www.ls1.com.au , then you may have read about the amount of movement there is when you press the brake pedal hard. There has been a lot of activity on the forum about this and even more about additional bracing being used to reduce the movement. So I decided to check out how much movement there was in my VZ-SS.

How I Did It...

Given the amount of scepticism surrounding this topic I decided to pull out the camera, mount it on a tripod and take pictures of any movement. I used my trusty Canon 20D with the 17mm-85mm IS USM EF-S lens. Mounting it on a tripod meant there was no chance of the camera moving between shots and messing up any differences. I also used a remote trigger for the camera for 2 reasons..

  1. it added to the stability of the camera.
  2. I was on my own so unless I used the self timer, it would be a bit hard to press the brake pedal and take the shot.
If you are into photography at all, you may be interested in the settings used... Just before we move on I should mention that I did run these through Photoshop. The only work done was resizing the images and colour / light balancing to match the images up ( and only a really quick job at that ). There is no cutting and shutting in these shots. If anyone wants the RAW files send me a PM in the forum, they are 8MB each.

Results

Well I guess I'll just show the pictures. Figure 1 shows the before and after shots of the fluid reservoirs during relatively hard braking. Now not I used the term "relatively", I'm guessing that the pressure I used on the pedal would not have been enough for the ABS to kick in. If this happens, then you loose all feel in the brakes anyway. Of course it's going to all depend on the amount of traction you have at the time. The thin is though, that the movement starts with minimal pressure, so even moderate pressure has movement.

Figure 1

Move your mouse over the image to see the difference. (more comments below the image.)

I think many will be surprised at the amount of movement there is in a car that is just over 1 month old...
I don't have one of the braces mentioned in this thread so I can't say what the difference would be, but just looking at the standard brace I'd say it would easily reduce the movement.
Some may say that it's only about 1.5 mm of movement, so who cares? Well the issue is that you are seeing the amount of movent at the master cylinder, not at the pedal. The lever action of the pedal results in an increase in the amount of movent. I did some measuring, but it's pretty hard to get accurate figures without actually pulling the pedal off. So take the following with a grain of salt...

So doing some the maths on this shows that the brake pedal is actually a 4.4:1 lever ( 330 / 75 = 4.4 ), making the 1.5mm movement of the master cylinder feel like 6.6mm at the pedal ( 4.4 * 1.5 = 6.6 ). This is almost the same amount as the free play in the pedal. The way I see it is if can feel the free play in the pedal, you'll notice the difference between a "moving" master cylinder and one that doesn't.
So am I going to test a brace? If I get one to fit the VZ I'll try it, but at the moment I don't really want to buy one and put it on a brand new car that is under warrantee... But if someone wants to loan one to me for a day, I'm always contactable via PM....