by George Nenadovich
A lot of 1968-72 Skylarks came from the factory with drum brakes which are not the greatest brakes. In 1970, the GS455 could be ordered with drum brakes. In 1971, all factory big block cars came with disc brakes. It was mandatory. I guess GM found out that the drum brakes were not capable of stopping the big block cars in a safe distance and were prone to drum brake fade. In 1973 and later cars, they all came with disc brakes as standard equipment.
In order to change a drum brake car to discs, several items are needed for the conversion. First, a donor car needs to be located with factory installed disc brakes. You will need to remove the spindle assembly (spindle w/backing plate, caliper, rotor) by separating the two ball joints. You will also need to remove the brake hose bracket from the frame. The drum brake and disc brake brackets are different. If you forget to remove the brackets, you can make the drum brake brackets work by filing the brake to the shape of the brake hose metal end.
Second, make sure you remove the proportioning valve from the frame which is located just below the master cylinder. The drum brake cars have a rectangle shaped valve while the disc brake cars have a very irregular shape. Once you see the difference, you will understand what I mean. You can check some cars at a show to see the difference.
After you get all the parts, inspect/check all parts. Make sure the rotor meets the minimum thickness requirements which is approximately 1.125 inches. On the rotor, the minimum thickness is usually cast into the rotor. Also, factory rotors have a rain groove in the middle of the braking surface which is approximately 1/8" deep. If you don't see this groove, the rotor has been turned too many times and should be replaced.
Also, check the bearings and races which are pressed into the rotor. Make sure the bearings are not blue(overheated), nicked, broken, galled, fractured etc. Inspect the pressed-in races in the rotor for similar damage. If the bearings are damaged, check the spindle to see if the bearing surface is also damaged.
With all the parts from the donor car, BUY a NEW master cylinder. These are available from Raybestos and TRW. I have had a lot of problems with rebuilt cylinders and the price difference is about $20. I also like to use silicone brake fluid which is more expensive than DOT 3/4 fluid. Silicone lasts longer, does not harm the paint or brake hoses and is not hydroscopic(absorbs water). Silicone will stay clear and does not become murky like DOT 3/4 fluids.
As a final wrap up, here are some costs with doing a conversion:
New Master Cylinder $40-$50 New inner & outer wheel bearings $10-15/pr New bearing seals $1-2/ea New brake hoses $25-35 each Rebuilt brake calipers $10-20 ea New rotors $40-50 ea. Proportioning Valve when available new(discontinued?) $80, rebuilts ?
The power booster does not have to be changed when doing the conversion. When doing the conversion, it would be best to do the suspension at the same time. New bushings, ball joints, idler arm, center link, tie rods springs and shocks should be installed if not done previously. Once you complete the front end, you will be ready for another 50,000-100,000 miles of driving. I recommend AutoDrive suspension parts made by Moog, and springs are available from TRW and several others.
Cars that you can use disc brakes from:
1968-72 GM A-bodies which are Chevelle, Cutlass, LeMans and Skylark 1970-72 Monte Carlo and Grand Prix 1970-81 GM F-bodies (Camaro/Firebird) with adapter kit sold by several aftermarket companies, expensive. Note: some early 1968 cars used four piston caliper disc brakes and these are rare. Most use the single piston design. Single piston calipers can be found on a lot of GM cars. Four piston calipers are found only on 1965-67 GM cars and all Corvettes from 1965-82.
If you have any questions, you can e-mail me. George Nenadovich