Rebuild of brake calipers - 1985 K100RT

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Rebuild of brake calipers - 1985 K100RT

Postby Terry » Wed Nov 19, 2003 6:36 pm

I have a 1985 K100 RT with leaky front brake calipers. I bought the O-ring kit, but I have no instructions on how to install it / rebuild the brake calipers. The Haynes manual I have does not cover this.

Has anyone done this, or have instructions on how to do it??

Thanks in advance


Postby Ted » Thu Nov 20, 2003 2:05 pm

You might want to drop my good friend Don Eilenberger [] a note, he knows K100's inside and out and I believe has done this exact procedure.

That said, I can't think that the process is a whole lot different than doing it with older BMW cage calipers. Does the rebuild kit look anything like this (with seals and boot)?

Found this, might be helpful as a reference point (for an e36 M3 cage):

We’ve all done it before, so let’s just admit it right up front. The last time you did your brakes, all you did was push the pistons back in their bores, slapped in the new pads, and called it a brake job. Now, assuming that the rotor did not have scores the size of the Grand Canyon, and were not the factory originals, you WILL have brakes.

But what happens when you push back the pistons, and one of them doesn’t go? Or the rubber boot rips? Or you come out one day and find a pool of brake fluid on the floor? Or you decide you’ve had it with that pull to the left the car’s had since you bought it?

Then you have to get serious and REBUILD your calipers. Or you could buy new ones, but at about $2-300 apiece, it could get a little expensive. Rebuilding, on the other hand will cost about $50.00. And you will have learned something as well.

The front calipers have four pistons and the rears have two. You’ll need to obtain the rebuild kits for them. Fortunately, they are inexpensive.

NOTE 1 If you’re going to all this trouble, you might as well do both front and back.
NOTE 2 You should also change the fluid as well. If you are planning on going to any driving schools, fluid changes are mandatory. You would be amazed what new fluid will do to old rubber parts.
NOTE 3 It may be a good time to replace those flexible hoses. They crack with age and are the most common cause for the classic pulling to one side syndrome. They are readily available, and you might consider the braided racing hoses, which are actually less expensive than the original rubber ones.


First thing you have to do is remove the calipers, by removing the two 19mm bolts, but before you do this, remove the brake lines and the various bleeder nipples. It is much easier to work with them when the caliper doesn’t move. Take care not to break off the nipples.

Now that the caliper is off the car, it is necessary to remove the pistons. This can be a little tricky, so be careful. You can blow out two of the pistons very easily by gently applying compressed air to one of the fitting holes on the caliper, emphasis on the gently. And make sure your fingers are not in the way, because they tend to pop out at light speed. Brake fluid will also be flying all over so make sure you’re wearing safety glasses and have any painted surfaces well covered.

Of course the pistons that pop out will be the loosest ones. The other two are going to be a bear. Your best bet is to put the two that popped out back in and hold them in with some vise grips. Now apply the compressed air again to pop out the other two. If you’re lucky they’ll pop right out. But you don’t want them to come all the way out, let them sneak out most of the way and then remove whatever you have holding the others in place. You want all four to come out most of the way, so you can finally pop out two, and have room to get a pair of pliers on the other two. Be careful with the pliers. You only want to grip the head of the piston. Do not grip the body of it. Twist until it pops free of the bore. If you can move them with your fingers, use them instead.

So much for fantasy land. What about the real world. No matter what you do, one or two of the pistons won’t come out. (If all four pop out of both calipers, be sure to play the lottery, because you are definitely on a roll.)

The first thing you need to determine is whether or not the piston is frozen. If the compressed air will not budge the piston, you can bet it’s frozen. It’s time to try the Channelocks. As mentioned above, grip the piston by the head, not the body. If it’s all the way into the bore, you may not be able to get it out, but if it’s sticking part way out, you should be able to twist and wobble it out. If you really can’t get the piston out, you’re going to have to replace the caliper. Get one from a junk yard, and make sure that all the pistons move, and rebuild it.

OK, now you have the pistons out of their bores, carefully remove the boots and their retaining rings, and then you have to remove the rubber seal inside of each bore. Use a small poker or flat screwdriver, but don’t score the bore.

Do not under any circumstances split the calipers. For some reason, once they are split, even using new seals, they always leak.

The next step is to thoroughly clean everything in solvent. Once the caliper is clean in and out, you must prepare the bores and polish the pistons. Use a Scotchbrite pad or some other semi-abrasive. Clean, don’t score or sand the things.

When everything is ready, clean everything once again but use alcohol this time. The alcohol will help remove any solvent left in the passageways, but won’t destroy the brake fluid. Blow out all the holes, and let everything dry.


First, wash your hands. With brakes, EVERYTHING must be clean. Lubricate the bores with clean brake fluid. The easiest way to do this is to dip your finger in the fluid, and wipe it around the bore. Now take the seal and place it in it’s groove. This is easier said than done, but with some coaxing it will eventually seat itself properly. Do this for all four bores.

Now it’s time to insert the pistons. Coat everything with brake fluid again. Install the boots onto the pistons BEFORE inserting them. This makes it MUCH easier to install the boots. Gently slide the piston into the bore. If there is fluid on both the seal and the piston, it should slide right in. If you have to force it, something is wrong. Try re-aligning it. Once the pistons are all the way in, you can install the boot retaining rings.

The calipers are ready to be re-installed on the car.
In eight years of owning BMW’s I’ve never been able to figure out why. I’ve even talked to mechanics that have forgotten more than I know about these cars, and everyone says, ‘Oh yea, don’t worry about it.’ So I haven’t, and it’s been a year now since I rebuilt my rear calipers, and stopping is NOT a problem.

From here on in, you can go back to your old way of doing brake jobs.
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brake rebuild

Postby Terry » Thu Nov 20, 2003 4:46 pm

Thanks for the info, but the kit I bought has no bolts, etc. It just has about 4 o-rings and some special grease.

Postby Anton » Mon Nov 24, 2003 12:21 pm

Terry said: "Thanks for the info, but the kit I bought has no bolts, etc. It just has about 4 o-rings and some special grease."

The basic's of Ted's procedure still apply. Your bolts get re-used.

You say "four O-rings." Are these actual O-rings or are they the v-profile seal? The piston seals will be about 1.5" in diameter and have a thin edge on the outside. There might also be two small (about 3/16" across) O-rings for the caliper joint.

The article mentions "don't split the caliper" but I did and had no problem. Other than that, just apply the same technique. Use air if needed to blow the pistons out, clean the bores, assemble piston and seal, reassemble, etc. Make sure the small O-ring between the caliper halves is not damaged. I assume the special grease is for lubricating the seal on reassembly, but I'm not certain. I never used any. Use a torque wrench to get the right torque on the bolts holding the two caliper halves together!! Do not shortcut this part!! Clean and dry threads, as usual.

Any problems, post here. It's not a hard job as long as you can get it disassembled.
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