“Disc Brakes Continued”
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Not being a brake guru by any means I really can't tell you why certain combinations of brake components work and others don't. I have very little understanding of fluid pressure and piston bore sizes either. What I can tell you is a good brake system is balanced, you don't want the rear brakes locking before the front and you want the pressure your foot applies to the pedal to be translated to stopping power. At this point in my conversion to C5 front brakes I am in the perfect position to play a little and see just how some different components don't work well together.
The first scenario is the big C5 rotors and calipers up front and factory drums out back connected to the 1 1/8" bore master cylinder and power brake booster just like my stock style single piston disc/drum setup was. The results were disastrous to say the least. I had 100% rear lockup with any more than the slightest of pressure on the brake pedal. The car was not drivable in this configuration. Next I added an adjustable proportioning valve in line with the rear brakes. Wow what an experience that was!! I learned that the pressure put out by a 1 1/8" bore master cylinder just wasn't enough to operate the dual piston C5 calipers. I could adjust the proportioning valve to reduce line pressure to the rear anywhere from locking up as described above to the point the rears didn't work at all. In doing so that's where I learned there was not enough pressure being generated to work the big front calipers.
My next option was to bolt up the 1" bore master cylinder that came with the kit I bought. This is where I found out manual and power brake master cylinders use different length and diameter pins and the one sticking out of my booster wouldn't match up to the master I had. No problem really the kit I had was designed to be manual anyway so I just moved forward and removed the booster from the car. To acomplish this as well as hook up the manual master to the brake pedal requires some time under the dash. The clevis and rod attaches in a different position on the pedal arm than the power setup does. The biggest issue I ran into was getting the pedal positioned so it wasn't too far off the floor and getting the brake light switch adjusted to work properly. Check the AIM (Factory Assembly Instruction Manual) for your year car to see the differences. Once past this small challenge it's on to brake bleeding and some road testing and adjusting of the proportioning valve.
I don't really have any real safe open roads near my house for brake testing but lucked out as the day I was doing all this was rather dismal and it was just starting to sprinkle when I finished bleeding the brakes. This very light rain left the streets nice and slick and made for the perfect low speed test environment. I putted around under 25mph and adjusted the prop valve until the rear brakes stopped locking up first and then took just a little more out to insure the fronts had a chance to lock before the rears. The rain really started to pick up so since I had reached a point I felt the brakes were as balanced as I wanted them I put the car away and had a couple beers and spent the rest of the day treating the car to a good detailing. The next chance I had to drive the car the roads were nice and dry. At first I was a bit taken with the amount of leg it took to get the brakes to operate on the dry road surface and once the car was up to operating temps I figured a good way to test the brakes was to do a bit of a power brake. With my left foot firmly on the brake I applied pressure to the gas pedal with my right until the rear wheels were spinning. The idea behind this test was if the front brakes were not doing their job the car would over power them and begin to move as I gave it more throttle. To my great pleasure the rear tires spun and the car stood still the whole time.
Since then I have only had one other chance to really drive the car and at that never got going more than 50 mph. What I have realized is it's been a real long time since I have spent any time behind the wheel of a manual brake vehicle. I'm learning that at speed (all of 45-50 mph) it takes less leg to make the brakes work than it does below 30 mph. This is consistant with a conversation I had with Jonathan at Touring Classics. He suggested a smaller diameter bore master cylinder would lessen the leg power needed to bring the car to a stop. At this point it won't be too long before I install the Touring Classics rear kit I picked up and will hold off making a master cylinder change until I have disc's on all 4 wheels. I will be updating this article when I get the rear conversion finished and have more info to share...
Don't forget the TC Brakes and Suspension forum as a resource, the search feature is very handy for getting the answers you are looking for! Several team members have compiled disc brake conversion information on their personal web sites that I think will be of great value as well.
David Pozzi (davidpozzi)
John Malouf (torker)
Carl Casinova (CarlC)
Jonathan Duke (duke's68)
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