Another Brake Question
 

I have searched around and not really found an answer for the earlier C4's. Lots of info for the later cars like 90 and up.

So here it is. I have upgraded my 86 to C5 front brakes. They work great but I'm not really happy with the peddle travel. I believe that it is caused by the master cylinder. I went from small single piston calipers to the large dual piston C5. So there needs to be a lot more fluid flow and the 7/8" bore on the C4 master just doesn't seem to flow enough even with the new DRM bias spring. I want to know if I can upgrade my 86 with the master to say a 92 that has a 15/16" bore or a C5 with a 1" bore. Will it fit? Will it work with my sensors and ABS? Would I still need my bias spring? I would love to hear from someone that has upgraded an 86 master and what they went to.

I had done the front C5 brake conversion on my old '87 autocross car and I never noticed any difference in pedal travel from the stock brakes to the C5 fronts. I had also installed the Doug Rippie bias spring and that didn't change things either (outside of helping with the rear brakes). There have been a number of members that had done the C5 front brake upgrade but I don't recall people mentioning a change in pedal travel.

I looked at Rockauto part numbers for '86 and '87 master cylinders, they are the same and both have a 7/8" bore size.

When you installed the front calipers. was there any large loss of brake fluid? If so, you may have created an air pocket in the master. Maybe bench-bleeding the master would be worth trying.

The ABS unit is not easy to bleed so it may be worth doing that too. There is a bleed valve on the assembly but the unit cannot be bled using something like a Tech-1. It's done manually and can get messy in the compartment. But maybe bleeding the ABS may be a help.

I did a full bleed when installing the brakes except for the ABS. I did not know how to bleed that system. Is there a bleeder screw or something on the unit behind the drivers seat? The peddle travel is longer now then with the old brakes and it my just be a psychological thing because the brakes seem to work fine. I just like a real crisp, firm peddle that responds almost as soon as I touch it. I looked at Rock Auto as well and found that they changes Masters in 88 and 90 when they made the bigger brake on the late cars which are very close in size to what the C5 is. I was hopeful that someone would respond that they installed the 90 master with no issues and it works great :) but the answer is rarely that easy.

I put C5 brakes on my 86. made the caliper brackets and bolted it together. Even used the C5 hoses since they were 18 years newer than my originals. Also used the C5 wheels

No issues. no pedal change. Just stops faster.