After seeing some of the comments regarding C3 Brakes in this thread I thought it might be useful to look at the differences between lip seal vs O-ring seal pistons in C2/C3 disc brakes, for the benefit of anyone not already familiar with these differences. My source for much of what follows regarding lip seals is the January 1965 SAE paper ‘High Caliper Braking’ (by Duntov & Brown from Chevrolet, Shaw from Delco Moraine) on the development of Corvette disc brakes.
In any disc brake setup, one of the main components – disc, caliper or piston – must be able to move or ‘float’ in order to accommodate the movement of the hub on the spindle due to cornering forces etc.
I have never seen a floating disc on a car but I think these are common on motorbikes, and many cars on the road today have floating caliper brakes. For various reasons, when GM/Delco were developing the C2/C3 disc brakes they decided to go with fixed discs and calipers, and use floating pistons to absorb any movement. Another requirement was for the pads to be in constant contact with the disc to minimise brake pedal travel.
For floating pistons to work, they need a mechanism that enables them to keep the pads in contact with the disc as it deflects on the spindle due to cornering forces, and/or due to disc runout. Without such a mechanism, the pistons will be ‘knocked back’ by the movement of the pads, displacing brake fluid back to the master cylinder. The effect of this will be to require additional brake pedal travel on brake application because the displaced fluid will first have to be pushed back into the caliper before the pistons can make the pads contact the disk. When driving, this can be experienced as a slight lag between hitting the pedal and starting to brake, plus an initial ‘soft’ feel to the pedal.
GM/Delco solved this problem by using springs behind the pistons to keep the pads in contact with the disc as it deflects on the spindle. The springs need to be light enough to avoid creating excess drag, and the piston seals must have less friction in the cylinder bores than the spring force for the springs to do their work. The flexible lip seals used in original C2/C3 brakes meet this requirement and eliminated the knock-back problem.
However, good as the original C2/C3 disc brakes were, a couple of problems started to appear over time. One was a tendency for moisture to enter the cylinders and start corroding them as the piston boots degraded. The other was a tendency for the cylinders to leak if cars were not used for extended periods of time.
The first problem is beyond the scope of this article. The second, the leak over time problem, was due to the flexibility of the lip seals allowing the pistons to drop in their bores due to lack of use. This would deform the seals just enough to allow fluid to pass them. Hence the advice you will often see to ‘tap the brake pedal regularly’ if the car will not be driven for some time.
To combat the leakage problem, after-market suppliers developed O-ring pistons as direct replacements for the lip seal pistons, and many C2/C3 owners now use these. There is no doubt that O-ring seals provide much better leakage protection than lip seals during periods of inactivity. However, there is a possible disadvantage to using O-ring pistons in that the O-ring seals are a tighter fit in the cylinder bores than the lip seals. Consequently, if an O-ring piston is ‘knocked back’ the springs may not be strong enough to overcome the seal friction and enable the pistons to keep the pads in contact with the disc.
After doing considerable homework on Mr Google, I think the lip vs O-ring consensus would seem to be as follows:
- For cars that will see limited use, or will spend much of their time on display, O-ring pistons will be superior due to their better ability to prevent leaks over time.
- For cars that are driven regularly, O-ring pistons should work well in cases where bearings are correctly adjusted and disc run-out is a minimum.
- For cars that are driven regularly but which may have looser-than-spec wheel bearings and/or excess disc run-out, there is a good chance that piston ‘knock back’ will occur, resulting in a hesitation when the brakes are next applied.
Chris Sale
’64 Coupe (with lip-seal front disc brakes)
