Brake Issues, Yikes! - Lateral-g Forums

ProTouring442 · #1 07-31-2008, 06:13 PM

Help!

Ok guys, this is the set up:

1956 Chevy convertible. We cut the front frame and installed the front sub from a 1979 Firebird. We also used the 8.5 rear from the same car.

The front rotors, calipers, and suspension is stock 1979 Firebird, all new.

The rear drums, wheel cylinders, etc are stock 1979 Firebird, all new.

The power booster, master cylinder, and combination valve are stock 1979 Firebird, all new.

The problem:

The car lacks brakes. Normal stopping seems ok, but you cannot lock either the front or the rears, no matter how hard you push the pedal. The pedal travels near to the floor when pushed very hard, and cracking either the front or rear lines loose allows the pedal to drop further, so it seems to me that nothing is bottomed out.

A little more info... I just took the car for a ride, and I can get the back brakes to lock, maybe the fronts, but only with an incredible amount of pressure. I am starting to thin it might be a pedal ratio or a master cylinder size issue.

So where do I go from here? Any thoughts?

Thanks!!!!!!!!

Shiny Side Up!
Bill

Vegas69 · #2 07-31-2008, 06:58 PM

Did you bench bleed the master cylinder? Sounds like a possible leak somewhere as well. Check for leaks and rebleed.

ProTouring442 · #3 07-31-2008, 07:45 PM

Quote:

Originally Posted by Vegas69

Did you bench bleed the master cylinder? Sounds like a possible leak somewhere as well. Check for leaks and rebleed.

Yes, the system was very carefully bled, and has been on the road for a bit, no leaks.

A little more info... I just took the car for a ride, and I can get the back brakes to lock, maybe the fronts, but only with an incredible amount of pressure. I am starting to thin it might be a pedal ratio or a master cylinder size issue.

Thanks!
Bill

Vegas69 · #4 07-31-2008, 09:00 PM

Pedal shouldn't sink to the floor regardless. Is the booster working properly? Pedal ratio is hard to believe with a power assist but only you know how it's set up.

Apogee · #5 07-31-2008, 10:26 PM

If you can measure the pedal ratio, that would certainly be helpful information to know. There are a couple of ways to increase brake torque without upgrading to larger rotors and/or calipers, but if you wanted to assume that it's the pedal ratio that is lacking, you could always decrease the MC bore size to generate more pressure at the calipers relative to pedal input.

What pads are you running? Increasing the coefficient of friction of the pads is another good way to increase brake torque, but it comes with a price. You'll eat rotors much faster and get to deal with dust and noise that comes with the higher coefficient of friction, so not necessarily ideal for a street car. I've run some Napa pads in the past that felt like glass on the rotors...easily the worst stopping, longest lasting pads I've ever run on anything (and I'm ashamed to have run them as long as I did).

Tobin
KORE3

ProTouring442 · #6 08-01-2008, 08:26 PM

Quote:

Originally Posted by Vegas69

Pedal shouldn't sink to the floor regardless. Is the booster working properly? Pedal ratio is hard to believe with a power assist but only you know how it's set up.

I must not have expressed myself properly, because the pedal doesn't "sink" at all. I can push the pedal near to the floor, but only with the engine running (full boost) and only with a great deal of effort. Even then, the pedal is not fully to the floor, as cracking the lines loose will cause the pedal to sink, as it should, but also indicating that the master cylinder is not bottomed.

Thanks!
Bill

ProTouring442 · #7 08-01-2008, 08:32 PM

Quote:

Originally Posted by Apogee

If you can measure the pedal ratio, that would certainly be helpful information to know. There are a couple of ways to increase brake torque without upgrading to larger rotors and/or calipers, but if you wanted to assume that it's the pedal ratio that is lacking, you could always decrease the MC bore size to generate more pressure at the calipers relative to pedal input.

What pads are you running? Increasing the coefficient of friction of the pads is another good way to increase brake torque, but it comes with a price. You'll eat rotors much faster and get to deal with dust and noise that comes with the higher coefficient of friction, so not necessarily ideal for a street car. I've run some Napa pads in the past that felt like glass on the rotors...easily the worst stopping, longest lasting pads I've ever run on anything (and I'm ashamed to have run them as long as I did).

Tobin
KORE3

I believe the pads are GM.

Comparing the set up to my mother's '57, I am thinking that it is a pedal ratio/bore size issue. Her car runs a similar caliper (though I do not know that they are the exact same) but the push rod connects much further up the pedal as compared to my dad's '56. I would think that this difference might be enough to cause the pedal to hit the floor though, so maybe I actually need a larger bore master cylinder, but with the same pedal ratio as my mom's car.

When I go back to NC again, I plan on playing a few "swap the parts" games to see what happens.

Thanks!
Bill

P.S. Tobin, are the Kore3 rear brake brackets to fit C6 Corvette Z06 rotors and calipers to a Ford 9-inch available yet? I didn't see them on your website.

**Ummgawa** · #8 08-01-2008, 10:31 PM

Sounds kinda simple, but, os the rod that goes from the pedal to the Master Cylinder long enough?

ProTouring442 · #9 08-02-2008, 06:39 AM

Quote:

Originally Posted by Ummgawa

Sounds kinda simple, but, os the rod that goes from the pedal to the Master Cylinder long enough?

Nothing wrong with simple... sometimes we forget to "go back to the basics!"

The free play at the top of the pedal travel is definitely in the normal range, nothing excessive. Besides, the pedal is not bottoming, so even if it were a bit short, I still should have more stopping power than I do.

Thanks!!!!

Shiny Side Up!
Bill

Apogee · #10 08-02-2008, 04:06 PM

Quote:

Originally Posted by ProTouring442

I believe the pads are GM.

Comparing the set up to my mother's '57, I am thinking that it is a pedal ratio/bore size issue. Her car runs a similar caliper (though I do not know that they are the exact same) but the push rod connects much further up the pedal as compared to my dad's '56. I would think that this difference might be enough to cause the pedal to hit the floor though, so maybe I actually need a larger bore master cylinder, but with the same pedal ratio as my mom's car.

When I go back to NC again, I plan on playing a few "swap the parts" games to see what happens.

Thanks!
Bill

P.S. Tobin, are the Kore3 rear brake brackets to fit C6 Corvette Z06 rotors and calipers to a Ford 9-inch available yet? I didn't see them on your website.

A larger bore MC is going to generate less pressure, not more. If you have the opportunity to pick up a caliper pressure gauge, they can be useful in situtations like these where it sounds like you're not getting enough pressure in general. Let us know what you learn from swapping parts though as that could definitely shed some light on the situation.

As for rear 9" kits, those have been available for a while now. They're listed on our site at the following link in all of the various configurations (305mm C5/C6, 330mm C6 Z51, 340mm C6 Z06) with and without drum-in-hat parking brakes (DIHPB): http://www.kore3.com/products.php?cat=12

Even better, Scott put up a thread with his KORE3 C6 Z06 DIHPB kit installation with lots of pictures: https://lateral-g.net/forums/showthread.php4?t=14127

Tobin
KORE3