Master cylinder/brake system help

[blown69nova]

I am having a problem figuring out the proper master cyl. for my system.
I'm using stock d52 calipers-front and explorer calipers-rear with a wilwood combo/prop valve. These are manual brakes.
Front pistons= 2.94" (single piston)
Rear pistons= 1.81" (single piston)
Pedal ratio= 6:1
I tried the 1-1/18 mopar master and of course the effort was very high. Next, I tried the 67-76 Vette master and could not get a pedal- the first circuit felt spongy and the second felt like I hit a stop.
So are these calipers a mismatch, and could you guys help me out with a master?
Also, this is a 9 sec. drag/street car
Thanks, Steve

[Apogee]

Steve, you have somewhat of a mismatch, but not so bad that it won't stop the car. The fact that this is a 9 second drag car has me a little concerned, but mostly just due to the likelihood that you're running some itty-bitty front drag skinny that isn't capable of handling much brake torque prior to lockup, especially at 140+ MPH.

With a 6.7 square inch front piston area and 2.6 rear, you would probably benefit with less front piston area (Wilwood D52 might be a good choice with 6.28 square inches) or more rear, but if you were to swap to an aluminum front caliper then you could shed some excess weight off the car at the same time and go even faster.

When you tried the 1" bore MC, did you make sure that the MC was fully bench bleed? It sounds like you have air in either one or both circuits still.

Tobin
KORE3

[Skip Fix]

I just put a 1.125" Mopar style on after having poor pedal and too much travel with a Vette 1"(possibly a bad MC) with Wilwood Dynalites on the front and 2.5" GM metrics(same size as the 79-81 TA rear calipers) on the rear similar pedal ratio. A little pedal effort but tight and decent travel. They do make a 1.032(as well as Summit) and there are 24mm Mopar style out there for a 95 Caravan V6, but to get a 10-15% difference for me to swap smaller I don't think I would really feel that much on the pedal force, and a 2" pedal travel only increasing .2" don't think I would notice it either.

As Tobin said be sure it is bled well. When I bench bled my Strange/Mopar I would think I was done no bubbles and hard to push, then would go one more time and bubbles in it.

Another thing to look at is being sure since you have big front calipers using the front port on the MC as often they move more volume-I'm using it for the rear on mine as they have more piston diameter.

[blown69nova]

Thanks for the replies.
I do think there was air in the master and that is why the pedal felt so bad.
As far as the Wilwood d52 calipers go, would you use the 2" or 1.25" pistons?
Thanks, Steve

[Apogee]

Thanks for the replies.
I do think there was air in the master and that is why the pedal felt so bad.
As far as the Wilwood d52 calipers go, would you use the 2" or 1.25" pistons?
Thanks, Steve

For a front application, you would definitely want to use the 6.28 square inch caliper with 2X-2" piston diameters, the smaller 2.46 in^2 calipers are intended for rear applications.

Tobin
KORE3

[woody80z28]

I just put a 1.125" Mopar style on after having poor pedal and too much travel with a Vette 1"(possibly a bad MC)

Where did you get your Vette master from? I am under the feeling that my master just is not right. I have bled and bled and bled and bled and still have a lot of pedal travel, even with small pistons front & rear (2.38 & 1.75 single).

I got two from Napa and they both felt the same: like garbage. I've been looking at the Mopar unit as well. They are much better looking than the GM style - and maybe one would work!

[blown69nova]

Bingo, Napa!
I'm not sure mine is bad, but mine felt wrong too.
Steve

[Skip Fix]

Got my Vette one at O Reilley but was a new not rebuilt. The downside of a Mopar on a GM is the GM studs are wider so you have to grind on the MC slots to get it to fit.

My Vette had almost 3-4" of travel with little feel and built less pressure than the Mopar I am using now(put dual pressure gauges in the car to tell. It also looked like it bled down some on the pressure.

[blown69nova]

That's how my NEW Napa one feels.
I am going back to the Mopar one for now, and will play around with the rear bias until I can afford a complete upgrade.
Thanks again, Steve