Brake issue help!!

[1BAGTO]

I have a custom 1970 Pontiac GTO that was purchased from Barrett-Jackson about 4 years ago. The car has been in storage while my wife and were building a house and I am now getting around to making the car “mine” and fixing the things that I do not like about it.

The car is equipped with a Baer “Serious Street” brake conversion, a 7” CPP power booster, and a Wilwood 1.125” bore master cylinder. The engine is a little low on vacuum (between 10 and 14” hg) at idle. The issue that I am having is the car doesn’t stop very well and I cannot “lock” the brakes up no matter how hard I try. I have bled the brakes, checked for obstruction, etc. The research that I have done leads me to believe that the culprit is the Wilwood master cylinder as 1.125” bore masters are typically sized for manual brake applications.

Can someone please point me in the right direction as to what items I need to correctly fix the stopping issue? I have been looking into replacing the master cylinder with the new Baer remaster cylinder in the 15/16” bore size as this is the size recommended for the “Serious Street” setup (according to the Baer instructions provided in the vehicle’s build documentation). Also, is there a booster that is recommended for this application?

I appreciate the help!

Aaron

[DonohurJav]

I can point you in the right direction, but there are a few great brake experts on here that can hopefully chime in get you exactly where u need to be.

I can tell you that your 1.125 MC is NOT FOR A manual brake application. Larger masters are for higher pedal assist as they push more fluid but have a harder pedal thus requiring more help(assist/boost etc.).

With your current meager boost,or assist , you would be better with a 1", or 15/16 MC piston. Smaller size is easier to push with more pressure but less volume of fluid, but needs less assist.

Hydroboost generates the most pedal assist, followed by the electric master cylinders, followed by the vacuum boosters. Depending on engine vacuum of course.

Long story short, if you keep your large 1,125 MC, you need more boost, or pedal assist if you will. If you keep your vacuum booster, you will need a smaller MC. Likely 15/16-1".

Caliper piston volume plays a large role in what size master you need, then generally you match the booster needed to that.

There are several experts on here that can crunch the exact numbers for you.

Hope this helps some.

[79-TA]

DonohurJay summed it up well.

Going to the 15/16" booster sounds like the right move. Hopefully the 1.25" master was not chosen to correct some kind of brake fluid volume deficiency.

Here's a rough guide to vacuum booster output. It uses a constant 23 inHg, so it's not the best to represent your application. Your car might run out of vacuum before the benefits of a larger booster are fully realized. Note how much more effective 8" and 9" boosters are than a 7".


http://www.classicperform.com/PDFs/BoosterBrakePressureChart.pdf

[69L48Z27]

I run 9" dual diaphragm w/ 1" MC and it works great with the 6 & 4 piston Wilwood brakes.

[a67]

Is there a Baer rear disc set up also? If so it should be floating calipers. In that case you can go with a smaller diameter M/C. Fixed calipers will have knockback so the larger bore works better.

The difference is in what type of booster is going to be used, that will help define the M/C bore.

As for the 7" booster, it is worthless. Toss it in the trash.

So need to decide whether you want to use a dual 9" booster, or go hydroboost. The engine vacuum is on the low side to use a vacuum booster, so the hydroboost may be the better choice. In that case keep & use the 1-1/8" M/C.

If you do go with a 9" dual booster then a 15/16" or possibly a 1" M/C will be required. It would be best to get the caliper bore areas to help decide which M/C to use.

Of course can always go with a manual set up (no booster). This requires changing the pivot point on the brake pedal for the M/C rod, along with a small (7/8" ) M/C. And good brake pads.

Bob.