Truth on M5 caliper rebuilds...(sorry for length) (archive)

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As our car age, various things go wrong that you would not expect to have happen on a new car. Some of these problems most DIYers don't think twice about attempting, but there are some things that most people shy away from. For example, the average DIYer will have no problem doing a brake job, but when they find out that they have a stuck
caliper, many just think that their only option is to purchase a remanufactured unit through a dealer or Maxamillian. Well, this is not the case, BMW and Maxamillian sell what are known as caliper rebuild kits. These kits come with all of the necessary rubber O-rings to rebuild a caliper.
Now lets say you have a stuck caliper on your M535i, and you have decided that you want to attempt to rebuild your own caliper.

First, you need to remove the outer dust boot. Once this is removed you must remove the piston out of the caliper. To do this you can use one
of three ways. The first one is to just push on the brake pedal until it pops out. This is accomplished by having the rest of the brakes on
the car and working correctly, then you unbolt the questionable caliper from the trailing arm or strut tube and then you remove the pads. Once
this has been done, then you have a friend push on the brakes. You will see the piston start to move. Now you will want to have a catch can
under the caliper because as soon as you get the piston out the brake fluid that was in the reservoir and in the brake line will come out. Be
careful to catch the piston when it pops out because you don't want to damage it. This is an easy way of removing the piston, but you will
find that you can only do one caliper at a time and then you have to bleed the system before you can do a second one. It is also difficult to rebuild the caliper with it still attached to the brake line. So the second and third way involve removing the caliper completely from the car, this is also the preferred way to rebuild a multi-piston caliper like those lovely E28 M5 calipers that you just bought to put on your car. The second way to remove the pistons is to used compressed air. In the port that the brake line screws into, you pump compressed air into the caliper and this will push out the piston. This is the preferred way to do this step. Now, some of us who have rebuilt quite a few calipers have found that the fluid, and the compressed air won't
loosen the piston. Well don't worry, there is one more way of removing the piston. This involves using a vise grip and a vise. First you clamp the vise grip to the piston with as much force as you can. Now remember, you only want to clamp it to the unmachined surface. This is the area that is in direct contact with the pad. You will notice that it has a small diameter then the rest of the piston. Once you clamp the vise grip on, then you place the handle of the vise grip into the vise
and tighten it down. Now you need to grab the caliper with both hands and try to rotate the piston in the bore. This is made easier by plenty
of releasing fluid. Once you have gotten it to rotate in the bore then you need to carefully pull the piston out of the caliper by pulling and
rotating at the same time. The reason why you do both at the same time is because you want to make sure that you don't cock the piston in the bore. If you cock the piston you will find that you can damage the caliper bore. Now, people with the M5 multi-piston calipers will be asking, how do I get the vise grip on the piston. Well what you need to do is unbolt the two halves of the caliper. There are four bolts that hold the caliper together. Once you have removed these four bolts put them in a safe place because they are no longer available from BMW. You will also notice that there are two spacers, and four small O-rings. Non of these parts are available from BMW anymore, so it is important not to lose them. I suggest that once you have the small O-rings out that you soak them in NEW brake fluid. WHEN REBUILDING CALIPERS ONLY USE NEW BRAKE FLUID!

Now that you have the piston out, you open your rebuild kit. There is a larger rubber O-ring in the kit. This O-ring is actually placed in a
groove in the caliper bore. You need to remove the old O-ring. Then it is time to clean out the caliper. This is easily done with compressed
air, but can also be accomplished by plenty of brake parts cleaner. Once you have cleaned out the caliper you need to inspect the bores. If
you see any rust in the bore, you will need to clean it out. This is done by carefully and I mean carefully using some emery cloth on the
rust spot. Now that you have gotten rid of the rust spot, you need to inspect the bore for any scoring. If there is some slight scoring you
can probably smooth it out with the emery cloth, if there is deep scoring you will need a new caliper. Once you have cleaned and inspected the bore you will need to coat it thoroughly with brake fluid. Then soak the new rubber O-ring in brake fluid and install. Make sure that the seal is not twisted in any way. Now you need to clean any rust off the piston and also inspect it for
scoring marks. Once you have cleaned it and inspect it, you can cover it in brake fluid and carefully reinstall it in the bore. You must be
very careful doing this because you need to make sure that you don't cock the piston. But unlike removing the piston you cannot rotate and install the piston at the same time. You need to push the piston in by hand. You will know if the piston is cocked because it will become very very difficult to install. Now remember you will need to get the piston by the rubber O-ring so don't mistake getting the piston by the O-ring with it being cocked. Now that you have the piston back in you need to put on the new dust boot. Once you have that on, the piston caliper is back together. And
for those with the M5 calipers, you will need to bolt back together the two halves of the caliper. Make sure to clean everything and wet it down with break fluid. Also make sure you reinstall all of the small rubber O-rings and make sure not to pinch them. Once everything is back together, I suggest for your floating caliper to use high temp. wheel bearing grease on the guide bolts. This will ensure smooth operation of the caliper. Bolt the caliper back onto the car and bleed the brakes.

Now you have successfully rebuilt your caliper and probably saved yourself at least $100 for each calipers.

*The E28 M5 caliper is just like any other multi-piston BMW caliper and so the techniques can be applied to all multi-piston BMW calipers.

I have successfully now rebuilt 8 calipers now, four of which are M5 calipers. One set of these is currently on my 5 and has been through 1 two day driver's school and 2 auto-xs so far. I have gotten the pads to almost their boil over point on track, so I am not light on the brakes, plus I am running Porterfield R4S pads and SP8000 225 tires on the car. So I am generating quite a bit of heat. If the bolts were stretch bolts, then I would have had massive leaks by this point or have snapped the bolts. This is definitely not the case. Just because it is a shouldered bolt does not mean that it is a stretch bolt. Plus, if you know about materials science, you will realize that using a stretch bolt in an exceedingly high temp. application is pointless. This is beacuse when you stretch the bolt into the plastic deformation region you significatly change the strength characteristics of the bolt, the bolt is less resistant to temperature change under load and is more likely to fail. If you used a stretch bolt in this application then you would have fluid leaks whenever using the brakes hard and getting them hot. And remember that the calipers get hotter than the motor, so comparison is not valid reasoning. And brake part cleaner can be used during rebuilding because you will remove all of the rubber seals.

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