Overhauling brake calipers on an M635CSi
This article provides a step-by-step procedure to overhaul the M635 brake calipers i.e. clean the pistons and install new o-rings and rubbers. The article may also be of use when replacing brake discs or pads.
I got to this exercise after experiencing a noisy trembling at the front, at speeds above 90 km/h (which occur more often than not.....). Jacking the front up revealed that the left wheel could hardly be rotated at all by hand. I took another test drive and measured the temperature of all discs, which was 50 degrees Celsius for the rear, 110 for the right front but 160 for the left front disc.....
1) Preparation:
- overhaul sets (item 7 in the picture, dustcovers, springs and o-rings)
- optionally a new brake line kit. I used steel inforced brake lines
- ATE brake cilinder paste (Bremszylinder pasta)
- 7mm and 19mm socket wrenches
- 14mm open wrench
- prying rod (an 8 inch screwdriver will do)
- rubber cap to top the brake line
- hammer and punch (to drive out the spring pens)
- lots of cleaning towel....
2) Disassembly:
Remove the wheel
To drive out the spring pens....
....use a hammer and punch
With the pens and spring removed, you'll get a clear view on the pads.
To replace the pads, pry them out with a screwdriver or other rod.
The brake pad wear sensor may be removed first. In case the pads will be reused later, they're best put in marked bags (e.g. Left-Front-Inner).
The ABS sensors are best removed and cleaned with a steel brush and some brake cleaner; the magnetic tip tends to attract dirty metal particles.
3) Overhaul
It's time to inspect the caliper's state. In my case it's clear that the rubbers that protect the pistons from dust are worn and need replacement. This requires the caliper to be disconnected from the brake line. In case you only want to replace the discs or pads, you may remove the caliper and attach it to a shock absorber spring with a tie wrap, leaving the brake line connected. Remove the caliper by loosening the 2 bolts with a 19mm wrench. Remove the rubber dust covers from the pistons by removing the spring rings with a small screwdriver.
You may now bleed the brake system, or just disconnect the brake line and cap the nozzle (which I did). I didn't bleed the brake system just yet, just loosened the brake line with a half turn of the 14mm wrench. Brake lines tend to get stuck really tight, and once the caliper is disconnected from the strut it's kind of awkward to apply leverage to unscrew the brake line.
Tie the brake line, nozzle facing upward, as high as possible to the car, I tied it to the spring. Be careful not to spill any brake fluid on the car, the hygroscopic character of the fluid will for sure ruin the paintwork.
Now some improvisation is required to press the pistons out of the caliper. I started with some artistic woodworking, carving a nice cross that would fit in the caliper, leaving room for the pistons to escape their rusty hideout.
Next I found myself a friend in the possession of an air compressor, and with a lot of patience, WD40, pliers and some violence he managed to push the pistons out.
Once the pistons are out, remove the o-rings inside the cilinder. Polish the pistons with very fine waterproof sanding paper (grain 800). The cilinders are not recommended to be polished but I did anyhow, or at least I cleaned the debris until they felt smooth.
I did find replacement pistons from Walloth&Nesch, but since these were not exactly identical to the used ones, I decided to clean the existing ones as good as possible, without machining away any metal. I also decided to replace the brake lines with some steel re-inforced ones.
After that, the calipers were cleaned with a steel brush and painted with hammerrite. The struts were also brushed and covered with lovely hammerrite.
I got to this exercise after experiencing a noisy trembling at the front, at speeds above 90 km/h (which occur more often than not.....). Jacking the front up revealed that the left wheel could hardly be rotated at all by hand. I took another test drive and measured the temperature of all discs, which was 50 degrees Celsius for the rear, 110 for the right front but 160 for the left front disc.....
1) Preparation:
- overhaul sets (item 7 in the picture, dustcovers, springs and o-rings)
- optionally a new brake line kit. I used steel inforced brake lines
- ATE brake cilinder paste (Bremszylinder pasta)
- 7mm and 19mm socket wrenches
- 14mm open wrench
- prying rod (an 8 inch screwdriver will do)
- rubber cap to top the brake line
- hammer and punch (to drive out the spring pens)
- lots of cleaning towel....
2) Disassembly:
Remove the wheel
....use a hammer and punch
With the pens and spring removed, you'll get a clear view on the pads.
The brake pad wear sensor may be removed first. In case the pads will be reused later, they're best put in marked bags (e.g. Left-Front-Inner).
The ABS sensors are best removed and cleaned with a steel brush and some brake cleaner; the magnetic tip tends to attract dirty metal particles.
3) Overhaul
It's time to inspect the caliper's state. In my case it's clear that the rubbers that protect the pistons from dust are worn and need replacement. This requires the caliper to be disconnected from the brake line. In case you only want to replace the discs or pads, you may remove the caliper and attach it to a shock absorber spring with a tie wrap, leaving the brake line connected. Remove the caliper by loosening the 2 bolts with a 19mm wrench. Remove the rubber dust covers from the pistons by removing the spring rings with a small screwdriver.
You may now bleed the brake system, or just disconnect the brake line and cap the nozzle (which I did). I didn't bleed the brake system just yet, just loosened the brake line with a half turn of the 14mm wrench. Brake lines tend to get stuck really tight, and once the caliper is disconnected from the strut it's kind of awkward to apply leverage to unscrew the brake line.
Tie the brake line, nozzle facing upward, as high as possible to the car, I tied it to the spring. Be careful not to spill any brake fluid on the car, the hygroscopic character of the fluid will for sure ruin the paintwork.
Now some improvisation is required to press the pistons out of the caliper. I started with some artistic woodworking, carving a nice cross that would fit in the caliper, leaving room for the pistons to escape their rusty hideout.
Next I found myself a friend in the possession of an air compressor, and with a lot of patience, WD40, pliers and some violence he managed to push the pistons out.
Alternatively, you can buy a 6 dollar grease gun which comes equipped with a hose that will exactly fit the brake line connection of the caliper. You could experiment with oil or other fluid (maybe even water?) to press the pistons out. The mess will be bigger, but you'll have more control over the procedure....
Once the pistons are out, remove the o-rings inside the cilinder. Polish the pistons with very fine waterproof sanding paper (grain 800). The cilinders are not recommended to be polished but I did anyhow, or at least I cleaned the debris until they felt smooth.
When everything has been cleaned, sanded, polished the caliper must be reassembled. Apply a thin film of ATE brake cilinder paste (or as the Germans call it: BremsZylinderPasta....) to the cilinder wall and piston surface, taking care that you do not put paste to the piston's rim where the dust cover resides. Insert the O-rings in the cilinder. Gently press the pistons into the cilinder. Apply the dust covers, and insert the spring rings retaining them.
After that, the calipers were cleaned with a steel brush and painted with hammerrite. The struts were also brushed and covered with lovely hammerrite.
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