I'm planning on replacing the rear axle of my 85 z28 5 speed to a posi rear with disc brakes. Will I need to change anything in the master cylinder or should I just change the cylinder itself? If so what should I get/what do you recommend?

 

For a rear disc swap all you need is a proportioning vave from the disc rear if it is a 10-bolt. if its a 9-bolt rear you will also need the parking brake cables. Flares on the fitttings may also be different depending on model year. My opinion is a 89 9-bolt rear with the 11.655 dia rotors and the aluminum PBR calipers. Pictures are of rear brakes.

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Stupid question, where would I find the proporioning valve and how hard is it to install? If you have a picture of it that would be great.

Thanks

 

It would be the last valve on the rear line, and should be easily removable as long as you have the right size flare nut. Also, as blackz87 said, find out what kind of flares you've got on the brake lines, as they are not interchangeable. I think they should be double flares, but they might be ISO. The ISO will look like a bubble at the end of the line, the double will have a cone shape on the end. If the new system uses a different flare type, you'll have to get a flaring tool and cut the flare off the end of the line that goes into your hoses, and re-flare it with the correct flare. If that's the case, and you have any trouble with it, say so, there are plenty of people on here who've probably done it at least a few times, including myself.

 

The valve is the aluminum block under the master cylinder. It will have about 5 lines going to it. When you go to replace it check the configuration as the lines dont always go in the same spot.

 

Oh and I had installed a Borg Warner 9 bolt from an 88 in my 84 and it had cables that hooked to where the originals did and all I had to do was hood up the rubber hose to the car for the brake fluid. I did get my rear end complette and it was an easy swap.

 

Oh, my bad. I didn't realize they were already using combination valves on 85 F-Bodies.