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C Disc Brake Caliper Overhaul

September 24, 2010 | Troubleshooting & Repair

Text and Photos by Steve Clarke

When I bought my 65 C coupe it had been sitting indoors for 18 years. While essentially complete some parts had been removed and were either missing or stored elsewhere in the house (I found the gas tank in the attic). Eventually I found 3 of its calipers; apparently the right front had been lost. I was able to obtain an original from Gary at Parts Obsolete, one of our trusted vendors. While complete, all calipers were rusty and needed refurbishing.

Dismantling the calipers is fairly straightforward. In my case some of the bleeder screws required heating the caliper half in order to remove the screws without risking shearing them in two. Remember that as you remove the cross pipes and bleeder screws the caliper assembly looses its assignment as to the left or right axle. If it is important to you mark the caliper halves so they can go back to their original position. I do not recommend the factory procedure for removing the pistons from the caliper bodies. The Registry website describes a safer (albeit messier) procedure "Disc Brake Piston Removal - The Easy Way", using a grease gun to force the pistons out.

With the square O-rings removed from the cylinder bores all parts are soaked in solvent to remove the grease, grim, etc. Once the parts are clean and dry everything can be thoroughly inspected. I use a dental pick to clean out the square groove in the cylinder bore for the O-ring. My calipers were in good shape with just some surface rust, and in one area I could still see traces of the original yellow plating. The faces of the pistons had significant rust, and some of the pistons had rust breaking through the hard chrome on the sides. (See right hand piston in photo below)

I bead blasted the calipers and took them down to my plater for yellow cadmium plating, and ordered new stainless steel pistons from Zim's to replace the old ones (see left piston in picture). I also ordered caliper rebuild kits. The factory manual says you should replace all the cross bolts, washers, and nuts with new parts. This stuff is grade 12.9 so be careful where you source this hardware. I found what I needed at www.boltdepot.com. If you need new cross pipes you can get them from Stoddard, and probably from some of the other vendors; although NAPA has a very good inexpensive substitute, part # 813-1260 which fits perfectly once bent into the correct shape.

overhaul kit

Once you have all your hardware and parts together you are ready for assembly. If you have both the left and right calipers apart you will have to decide which caliper (left or right) you will be working with, because that will determine the position of the parts as you assemble the caliper.

Another decision you need to make at this time concerns the piston retraction mechanisms. There has been much discussion of 356Talk about whether or not they are needed, and the decision is yours, because some people leave them out with no apparent problems. I think ATE must have had a good reason to include them, so I remove them from the old pistons with a pair of circlip pliers. They are installed into the new stainless steel pistons in the same way.

You are now ready to assemble the calipers. ATE says in their instructions that you should use brake assembly paste, but I can never find it. So in the past I used brake fluid, however lately I have been using NAPA Sil-Glyde with good results.

Double check that the groove in the cylinder bore is completely clean. Lightly lube the O-ring with Sil-Glyde and insert it into its groove. It will seem to be too long to completely fit, but it will finally snap into the groove with a little persuasion. Now lightly lube the cylinder bore, retraction pin in the middle of the bore, and the piston sides. Insert the piston into the cylinder bore. It should slide in fairly easily until the piston hits the top of the O-ring. Chances are you will not be able to push the piston past the O-ring by hand. I

Use a 4" C clamp to push the piston down, but it must be done carefully and in small increments. If you just crank down on the C clamp you may tear the O-ring (ask me how I know). So just tighten the C clamp a little, then loosen it, and repeat. This allows the O-ring to relax and recede into its groove. Do this a few times and the piston will work its way past the O-ring without damaging it.

[Comment from Alan Klingen of The Stable] The 20° step angle on the piston face is required because the pads want to "tip in" to the forces of the piston so the pads would wear on top first. ATE made the piston contact the bottom first, hence the angle cut. It's hard to describe but the pistons make contact on the bottom of the pad and the top of the pad gets driven in by a "servo" action. This is like the leading and trailing brake shoe action.

Just before the piston face is flush with the caliper body you need to rotate the piston to insure even brake pad wear. This is accomplished using a special 20° gauge, factory tool P 84 (which is easily fabricated). The procedure is described in the instructions that come with the ATE caliper overhaul kit, and also in the factory manual on page ST 35.

With the piston correctly oriented, use the C clamp to push the piston the rest of the way into its bore. Now install the dust seal. I put a little Sil-Glyde on the inner surface of the dust seal. Stretch the inner hole of the seal over the raised surface of the piston face. The outer edge of the seal should fit nicely in the groove in the caliper body. Take the circular spring from the rebuild kit and snap it over the outer edge of the seal. You are finished with one half of one caliper.

The other half of the corresponding caliper is assembled the same way of course, except that the 20° gauge is angled the opposite way, so that when the caliper halves are put together, the piston face recessed areas are parallel to each other. If they are not, then one of the pistons has been incorrectly set.

The two caliper halves are mated with the four caliper bolts, split washers, and nuts. Align the caliper halves so that the machined surfaces in the brake pad well are flush. Torque the inner pair of bolts first, then the outer pair. The front calipers require 24.6 ft lbs, the rear calipers 13.0 ft lbs. Go ahead and install the cross pipe in the bottom holes, and the bleeder screws in the top caliper holes.

At this point I like to place the caliper next to its axle and double check that I have installed everything in the correct orientation. Remember again, that it is the orientation of the piston faces, the placement of the cross pipe and the bleeder screws that make a caliper a right or left part, but they can still be bolted on to the opposite axle if you are not paying attention.

Pat yourself on the back, pop a beer (or whatever your favorite libation) and take a break, only three more to go!

[(b>Comment from Alan Klingen of The Stable) The other issue is the difficulty of getting the brakes to bleed. Inside each piston is so many parts that it can be very difficult to get all of the air out, a lot gets trapped in the inside stuff. If you look at modern calipers the piston is in reverse with the cup side outward. The best solution is to pre-bleed the caliper on the bench and bleed it at all angles of the caliper to get the air out. You also must tap on the caliper with a steel hammer as you bleed to dislodge bubbles stuck to the walls of the caliper. The calipers can be a bear we have even tilted a car 35+ degrees on its side to move the air. We had a C that we could never get a good pedal but after it sat in the shop for a week it all came back!

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