Booster size and Vacuum w/ 4 wheel disc brakes?

[chr2002ca]

Just wanted to ask y'all a quick question. Anybody out there running a 9" dual diaphragm booster(or smaller) with Baer disc brakes front and back AND you are able to lock up your brakes at 10 MPH+? I have the Baer track system(2 piston in front, 1 piston in back) setup on my '69 Camaro, have a 9" CPP dual diaphragm brake booster and MC designed for 4 wheel discs, and brand new stainless hard and flex lines front to back and I can't get my brakes to lock up. The car stops okay, but it should definitely be better. I've bench bled the MC twice and bled the entire system so many times I could puke. I just can't imagine there could even be a single bubble of air in the system, and I've tried all the tricks, trust me. I pull about 13-14Hg of vacuum at idle(800), about 15Hg at 1100RPM, and then it gets over 17Hg past 1300 RPM. Maybe max's out around 18Hg. I would think that's enough vacuum combined with a 9" dual diaphgram booster to be able to lockup my 4 discs. I understand that it's obviously not safe to be locking up your brakes, but the brake system should at least be strong enough to be able to do it upon request at low speed.

So I'm just curious if anybody else out there is running a vacuum booster with 4 wheel discs and what kind of brake performance they're seeing.

Thanks in advance for your feedback!

And uh, please, folks running manual brakes or using Hydroboost, I've already heard all about that stuff so there's no need to reply! :) Thanks!!!

[Apogee]

What is the bore size of the master cylinder? Your Baer "Track" kits are essentially C4 Corvette components and the C4 Corvettes came with either 7/8" or 15/16" master cylinders depending on the year. The size of the vacuum boosters varied as well by application (ZR1 versus base model, etc), but CPP has several sizes of MC's, so if you're running a 1" or larger bore, then that could explain your braking performance.

Tobin
KORE3

[chr2002ca]

The Master cylinder that came with my Baer kit was a 1 1/8" bore CPP master cylinder. This was mated to an 11.5" original style Delco Moraine brake booster. Due to valve cover clearance problems on my big block, I never used the first unit and instead replaced that MC and booster with a 9" dual diaphragm brake booster which came mated with a new MC and 4-disc prop valve, all of which are made by CPP. I purchased my latest unit in person at their store after telling them about my setup. It was the one that CPP said I should use with my Baer disk brake setup and my 9" dual diaphragm brake booster. It also has a 1 1/8" bore.

So thats twice that I have received 1 1/8" bore Master Cylinder for my power disc brake setup. Are you saying that I have now been given the wrong sized master cylinder twice? That would be a bummer. Why would they sell me a MC and booster setup that wasn't capable of providing enough brake pressure to the very common brake setup that I have and described to them?

I appreciate the feedback. :)

[Apogee]

Why? That's a good question that I can't answer, but I can say that you're not the first person to be told by a parts guy that you need a 1-1/8" bore MC because you're running disc/disc on a car without taking the caliper piston area into consideration. Just because GM ran 1-1/8" MC's for years with it's power brake applications does not make it universally true, only true for the caliper combinations that they chose to run for years.

It's ironic because CPP makes and sells it's own master cylinders from it's own castings...they have 15/16 and 1 inch bore options last I checked. Reduce your bore size and you should see a huge improvement in your braking performance.

Tobin
KORE3