Master cylinder and booster questions

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malibudave

Greasemonkey
Mar 12, 2010
204
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Houston, TX
The stock single piston front caliper has 4.42 square inches of piston volume if using a 2-3/8" piston diameter.
Your dual 38mm piston front caliper has 3.51 square inches of piston volume.
That being said, a caliper with more piston volume will have more clamping forces regardless of piston count.

A smaller bore master cylinder creates higher line pressure, which in turn creates greater clamping forces at the caliper.
A larger bore master cylinder creates lower line pressure, which in turn creates less clamping forces at the caliper.

Make sure your rear shoes are adjusted out against the drum as this will cause a long pedal stroke. This is the easiest to check without spending money.

Are you running the stock prop valve on the frame?

If you are running your stock, 30 year old rubber lines, I would replace them with new rubber lines or stainless steel braided flex lines. Over time rubber lines will start to balloon with age. This will cause a longer than expected pedal stoke because of the ballooning.

If you are using the stock step bore master cylinder, may be your issue. Its pressure bore is 24mm , which should be enough for stock, front calipers, but its a step bore.
Being a step bore means its has two bores. A large volume bore (1.25-1.375"), which transitions down to the smaller 24mm for pressure. The step bore feature is only used for the front brakes. The front brakes came from the factory with LOW drag calipers and require a step bore master cylinder to function correctly. That was said to say this. Step bore master cylinders are finicky, have extra parts, and may function abnormally causing a long pedal stroke. They are also harder to bleed. All master cylinder should be bled off the car because of the angle of the firewall of a g-body may cause air to be trapped in the master.

If you want to test the current master cylinder to see if it holds pressure, you can use a line clamp and clamp off all three rubber lines (two in front, one in rear). The pedal should be rock hard with little to NO movement in the pedal. If there is excessive movement, there is either air in the master or lines up to the line clamps, or the master cylinder is bad.

Keep in mind that though aluminum calipers are lighter, they may be more flexible and can "clamshell" when pressure is applied to the caliper from the master cylinder. I don't know if this will be the case with your calipers, but this deflection may also cause a longer pedal stroke.

Like said in earlier posts, pads do make a difference. If this is a daily driver, the green stuff pad should work well and may give you a better "cold bite" than the yellow stuff pad. The green stuff just can't handle the higher heat that a yellow stuff pad can, but that shouldn't be an issue with a daily driver.

Let me know if you have additional questions.
 
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