front caliper question
 

Just replaced all four rotors pads and calipers. Went for a 16 mile drive, everything fine. Next day went for a short 3 mile drive, got home and front passenger side rotor was so hot it was smoking. Serious dragging going on! Where do I need to start?

cardsnapper,

This is most likely a problem with the rebuilt caliper. I have copied the Mercedes-Benz manual chapter on this subject into threads on this forum in the past, but there are others who seem to have fixed the problem with different solutions. I cannot figure out why they worked and there have been no reasonable explanations for them.

The problem, as Mercedes-Benz describes it, it that the piston to caliper bore seal, a square cross sectioned "O"-ring loses its "grip" on the piston. This seal ring sits in a groove in the inside diameter of the caliper cylinder bore and is compressed to a specific preload by the insertion of the piston. When you get the system assembled and filled with brake fluid, the act of stepping on the brake pedal creates a pressure that forces the piston to squeeze the brake pads against the disc. Very little fluid actually moves in the brake lines with each application of the pedal as the pads are barely off the surface of the disc at the beginning of the event. The piston to cylinder seal in the caliper sticks to the piston, and is deformed by the piston motion.

When you take your foot off the brake pedal, the springs in the mechanical linkage, and inside the master cylinder, move the pedal back to its rest position. If these springs are gone, or not functioning correctly, it is feasible that the weight of the pedal and linkage parts could bear on the master cylinder piston and maintain a light pressure on the system. In all probablility this would cause all four brakes to heat up though.

When the master cylinder returns to its normal position with your foot off the brake, any "sucking" of brake fluid in the system that occurs, draws fluid into the master cylinder from the reservoir. The release of pressure, however, does cause the deformed seal in the caliper to pull the piston back slightly, which is the ONLY mechanism in the system for pulling the pads away from the discs. If the pads are not coming back from the discs, this is the mechanism that has failed.

There is good reason for this system to work this way, and one of the best is that if the system depended on a few cc's of brake fluid to be pumped around the system every time the brakes were applied, the different brake line lengths and number of bends from the master cylinder to the different wheels/calipers would have the effect of adding a great deal of time delay to the activation of the brakes at each wheel, and it would make the application delay unique for each wheel. Which would make the car difficult to control as each wheel would start and stop braking at a time that depended on the configuration and internal condition of the brake line to it.

The brakes work by transmitting pressure, very quickly, and minimal flow of brake fluid. Yes, you have to make up for flexing of the brake caliper and swelling of the hose jumpers from the steel brake lines to the calipers, all due to the pressure. This is a very small fluid volume,however.

I have rebuilt calipers in the distant past for a 1967 250S, and out of ignorance did as little cleaning as I could get away with, and essentially only replaced the seals. It worked. If I had honed the bore and polished the outside diameter of the piston, I do not think I would have been so successful. Changing the geometry of the piston, cylinder and seal enough to cause the seal to lose its "grip" is apparently easy to do. And it will not necessarily show up as a failure in a production pressure test.

The M-B check for this condition calls for testing the "pull-back" distance of the pad upon releasing the brake pedal after applying the brakes at 5 bar pressure and 90 bar pressure (using a gage; that is about 75 psi and 1323 psi). If it is less than some minimal value, like 0.01 mm, or more than 0.15 mm at 5 bar, or 0.21 mm at 90 bar, there is a problem. The cure is to replace the piston seal.

I hope all this helps. Jim

Might also look into the rubber brake lines. They break down internally with age and collapse. This can prevent the fluid from returning to the master cylinder and cause a dragging brake.

...Also a follow up to Mike's reply....if you pinched off the rubber line at the caliper it's also possible to have permanently collapsed the line as well.....;)