Help with master cylinder size?

[CFster]

64 Dodge Polara 500 with:

Front: Wilwood 12.19" rotors with 4 piston fixed calipers - 4.80 sq in total bore size.

Rear: TSM 11.25" rotors with Eldorado single piston floating rear calipers - 3.23 sq in bore size.

I used this formula from Wilwood to determine the bore size:

"To determine the effective piston bore area of any caliper, you must first calculate the area for each piston bore found on one
side of the caliper. Use the formula " Area = ( bore x bore) x .785" for each piston bore size. Then, add the areas of all pistons
on that one side of the caliper to determine the total effective piston bore area."

I believe this means the rears have 67% as much bore size as the fronts - which somebody somewhere said was acceptable. 60%-75% I believe is ideal.

I'm running a Wilwood Master Cylinder with a 1.06" bore - manual brakes.

I'm also running a Wilwood adjustable proportioning valve.

I'm getting a pretty hard pedal - takes a lot of effort to get this thing stopped. With the prop valve opened all the way, I still can't lock the rears.

I understand that a smaller bore master will yield more pressure. Does anybody have any recommendations for size or type?

Thanks,
-Colin

[CFster]

Btt

[fuzzyonion]

If the pedal is not moving much before it gets hard, you could try going down to a 15/16" bore.
Your pedal effort and bore size will also be determined by the lever ratio of the pedal. I am unfamiliar with the Polara, but it sounds like it might be about 6:1, same as Camaro.

If you have a lot of pedal travel before it gets hard, you might have a different problem, and a smaller master would have even more travel.

As far as what type master to use, are you running an adapter to use the 2 bolt style MOPAR Wilwood master, (plastic resivoir w/ screw on caps), or another type?

[CFster]

The pedal is high and hard - not a lot of travel.

It has an adapter, from the old 4 bolt to the 2 bolt Wilwood (plastic resivoir w/ screw on caps).

I'm pretty sure I have the rear calipers adjusted up properly (e-brake lever adjustment, and bled properly). The entire brake system is new - lines and all.

Do you have any recommendations for a 15/16" bore master?

[David Pozzi]

What is the pedal ratio?

[CFster]

Hmm, you got me...

Somewhere I saw a method to measure that - I'll have to search for it.

[fuzzyonion]

https://static1.pt-content.com/images/pt/2005/10/pic01_brakefacts-1.gif
If you determine that a smaller master would help, there is a similar one to the Wilwood MOPAR (at least in appearence) which is 24mm (15/16"). It comes on 91 Daytona, 15" wheels, 4wldisc. There is also a 22mm (7/8") on the 14" wheel model.
I think the Wilwood master you have is just a regular Mopar 1 1/32" master from a full size that they adapted a pushrod to.

The 6.2:1 Camaro pedal seems to like the 15/16" for manual 4wd brakes and your specs, from what I've read. It sounds like basically a C5 front with 2.125" Caddy rears.

[CFster]

Thanks FuzzyOnion, for the info!

I'm probably going to give that 15/16" master a shot.