I need help with Master Cylinder

[CHILI442]

I need a smaller bore master cylinder and need help with the application.

Here's the vehicle specs:
'68 Olds 442
Vehicle has power brakes and a spicy cam that pulls about 13" of vacuum at idle
Front brakes: C5 'vette with 13" baer rotors and Hawk HPS pads.
Rear brakes: 11" Ford Exploder with factory pads.
No prop valve. I use a DPI racing Lock Resistant Dampener in line on the rear brakes. It's a valve that dampens and delays the rear brake on high pressure spikes (oh sh*t!) and keeps the rear from locking. It's been on there for years and worked fine when I had 12" 1LE brakes on the front.

Currently I'm using a '79 Trans Am master cylinder for 4 wheel disk. I'm not sure what size it is, but I'm guessing 1-1/16?

The pedal is really firm. It's fine for normal driving, but in a panic stop situation the pedal is hard as a rock and I have to give it everything I got. It scares me. I know I need a MC with a smaller bore to build more pressure. From what I've read, a 15/16" bore seems to work for most people. My problem is what application. I'm looking for MC that looks like what I have. A new style aluminum/plastic unit might save weight, but would look out of place under my hood, which has a stock restored look. I would be willing to settle for one of those ugly Corvette bath tub looking ones if I had to. I've heard the '78 Malibu has a 7/8" bore, but I think the lines are swapped front to rear, and I don't want to alter that. I have nice stainless lines and all.

Any ideas?

[L92Firebird]

Your setup seems simular to mine. I have c5 front and rear on my 69 firebird. Here is what Tobin at Kore3 emailed me when I asked the same question. I am sure he wont mind me posting his email. It might save him answering other emails.

" First off, if your ’69 Firebird was a factory power brake car, then you can run the OE single-10.75” power booster if you like. This booster has about the same effective boost as a dual-8” diaphragm unit and would work best in your application with the ceramic pads you purchased with a 15/16” [24mm] bore master cylinder, however it can also work well with a 1” [25.4mm] bore MC. The difference between the two is about 13%, with the smaller bore unit requiring 13% less pedal effort to achieve the same pressure as the 1” bore unit, but with 13% more pedal travel.

There are more OE options in the 1” bore size range with the most common, inexpensive, and easily attained unit being the 1968-1976 C3 Corvette unit. These however have a deep pushrod hole in the MC piston and will require a spacer when run with a short pushrod booster like the OE Firebird unit IIRC. You can make one and/or buy one, as I believe CPP sells them with their master cylinders…you’d have to ask if they sell them separately. They offer C3 knockoff MC’s for $59 new with the spacer in both 15/16” and 1” bore sizes…pretty tough to beat IMHO. While it’s not an OE part, you could always get the 15/16” unit now, and run a C3 Corvette MC (1”) in a pinch if you had to with the only difference being the 13% mentioned above.

As for off-the-shelf, OE options, you can run a 76-77 Malibu 15/16” bore disc/drum MC if you like, although it is set up for manual brakes and the rear residual pressure valve would need to be removed for running in a disc/disc application. There is a 24mm Mopar unit which is aluminum and can be made to work quite well, however you’ll need to slot the mounting holes slightly since the spread on the MC is 3.25” and the spread on the booster is 3.40”. These came on various Mopar applications from 1984-1995 such as the Caravan. The third-gen F-body master cylinder is a 24mm unit and designed to be mounted at an angle, and while it is aluminum, it has a snap-on plastic cap for the reservoir of which I’m not a huge fan. It’s also a quick take-up type MC, so it typically results in a shorter pedal travel due to its internal valving design."

[woody80z28]

Interesting...Tobin is good! I looked up the Caravan master and it looks like you could swap the plastic reservoir with the older Dodge truck reservoir that is flat and not angled. I actually have one of them sitting around unused and I think the black straight reservoir looks better.

[MonzaRacer]

You might try the manual disc brake master cylinder for same application. Raybestos number MC 39023, also take a look at the pedal ratio. Dropping the push rod farther away from the pivot a small amount may help also the manual MC is 1" bore. I am wondering if the "dampener" might not also be an issue. If you care to experiment try plumbing an adjustable prop valve into the rear line. possibly in place of the dampener. Adjust till you achieve solid braking, foot modulatable.
I take them out, make sure you have used them a couple of times but not heated them up a lot. Do a hard stop from about 50. Trying to find threshold braking point can be fun, but make sure you let the brake cool by driving after each stop.
In all honesty if you have a properly adjusted (ie balanced) brake set up should pull hard and you actually should be able to self modulate with your foot.
Also another big tip. DONT BE ONE OF THE BROTHER CRUISERS. Your legs should not be scrunched up so bad as to slow your reaction times YET your knee must never straighten and lock. The best position taught to me was legs about 30 degrees.
I have had several customers who couldnt get good results after a fresh brake job. I have had to teach my customers how to sit in cars.
I have them tell me they arent comfortable, yet after proper setting of the seat many say they have had less driving stress and can actually drive better.
A few points The master cyl above may not be small enough but honestly I cant see why if the pedal ratio is right and you have aggressive enough pads you should be able to hit the brakes from 60 and actually modulate a HARD stop up to the point of near lock up yet not actually lock up.
Also adjusting more and more pressure to the rear till they lock before the front, then back off say 1/4 to 1/2 turn or what ever increments your using.
I actually have scribed lines on the knobs and made one line heavier,, for reference.
My first check, pedal ratio, second: adjustable prop valve with out the dampener (maybe add it back AFTER fully adjusting the prop valve), then swap in a smaller bore master cyl. I have found a 7/8 bore master cylinder, not sure if it could be made to work but a 77 Monza 7/8 bore mastercyl might work, and they even can be had with manual push rods if you can find one. Raybestos number MC39027. There may be some sort of residual pressure valve that needs removed from rear brake port. Oh and the Monza ports are swapped from the T/A master cyl but same thread size.