Master cylinder size?
 

I finished my rear disc brake install, but the master cylinder/brake switch/residual valve seems to have given up the ghost. I have a soft pedal, even though the car stops better than it ever has before. I can't even powerbrake! The dash "brake" light stays on all the time.

So, i think my solution is going to be a new Wilwood master cylinder.

http://www.wilwood.com/MasterCylinderList.aspx

The question is what size the old one is, and what size should I get on the new one? 1 1/8" is the largest bore, that would probably put me through the windshield.... or would it?

If you have driven any other car or truck with 4 wheel disc brakes , they all seem to have soft pedal when applying brakes , see if you can find some info on 80 to 81 camaros or trans ams some of them came factory with 4 wheel disc brakes . maybe you just need the brake switch/residual valve and master cylinder from one of those cars to make it work properlly.

My CJ-5 has a similar system - F-450/Wilwood fronts, Eldorado rears, and it worked great until i put in 5.38's that push through the brakes.... There I can shift into 2nd (full manual valve body) and it won't push, but the Camaro is a 5-speed.

I talked to Wilwood techs, and the recommended the 1 1/8" bore master cylinder. They also recommended i get rid of the factory porportioning valve, and the residual valve. In place i will install a variable porportioning valve in the rear system to reduce the rear brake bias. It will require some dialing in, but the brakes should be awesome.

So, i ordered the Wilwood polished master cylinder from Poly Performance, and should be able to get it in in the next week or so. I'll put it in the CJ first to see how hard the install is.

did you change your master when you went to rear discs? you need a different size rear bowl now

No, I was using stock disc/drum from '69

The larger the bore of the master cylinder, the harder you will have to press the brake pedal to stop the car. The force transferred from the brake pedal to the brakes is controlled by the ratio of the master cylinder bore compared to the wheel cylinder or caliper piston bore. Consider the brake system as a lever. With the same amount of pedal movement a big bore master cylinder will move the wheel cylinder a greater distance than a smaller bore one, but will require more force against the pedal to get the required pressure at the wheels. On the other hand, a smaller bore master cylinder will move the wheel cylinder a shorter distance, and will require less force at the brake pedal to get the required pressure at the wheels.

that is your problem right there. get the right mater and you'll be happy with the results

When I upgraded my system I had a similar problem. I had a soft pedal, the front brakes worked great, but the rear did nothing and the light was on. It turned out to be air in the master cylinder. I re-bled it and then the rest of the system and everything was fine.

that "could" be one of his problems, but it's not his main one. disc/drum and disc/disc masters use different size bowls because of the fluid that needs to move through the system.

with his current set-up, the OP might be able to get a better pedal than he has right now, by messing with different components, but until he installs the proper master, his brake system will never work as it's properly designed.

this isn't something that has to be troubleshot with different possible fixes. the IS the fix he needs to do

no if's, and's, or but's about it