Rebuilding the K75 front calipers

One of my todo list items was to clean the calipers.  I remembered that a few years ago I had ordered caliper rebuild kits from Bob’s BMW to fix a pulsing brake.  I repaired that problem buy renewing the pads and rebedding them.    These kits contain all of the pins and seals you need to refurbish the caliper. So I figured I would actually use them.

First I pulled the left caliper. I had to pull the bleeder screw completely out in order to get an 11mm flare wrench around the hard line coupling.

The bleeder screw interferes with remove the hard line, remove it completely first.  The screws that hold the caliper halves together are likely to be very stiff, loosen them while the caliper is still mounted.

The bleeder screw interferes with remove the hard line, remove it completely first. The screws that hold the caliper halves together are likely to be very stiff, loosen them while the caliper is still mounted.

It was only after I had the caliper completely off that I realized the screws that hold the caliper halves together were frozen, so I put the caliper back on the fork and used a cheater bar to break the bolts loose.  On the right hand caliper I broke those screws loose before I removed the caliper from the fork. Once the calipers were split, I used some compressed air to pop out the pistons.

These are the old parts. The corrosion on the caliper screws is pretty bad, no wonder they were so difficult.  The seals don't look bad, but they are old.

These are the old parts. The corrosion on the caliper screws is pretty bad, no wonder they were so difficult to remove. The seals don't look bad, but they are old.

These are the new parts.

These are the new parts.

When I pulled the right caliper I got a nasty surprise.

It looks like at some time the rotor ground into the caliper housing.

It looks like at some time the rotor ground into the caliper housing.

The outer piston (on the left in the picture) took a significantly higher pressure to remove.  It appears that this piston was frozen and that at some point the PO allowed the pads to go completely bad.

The pistons both looked fine and the bores were very good. I couldn’t find an obvious reason for the frozen piston so I cleaned and reinstalled it with the new seals.

After I bled the brakes I went for a test ride and found that the freeing the frozen  piston demstrably improved teh braking.  I was actually able to lock up the front wheel (first time ever), which while not a great thing, at least showed the dramatically improved performance out of the calipers.

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