85 K100 Basic - frozen front brake

[rawhites]

This bike sat for decades, so I inherited it with lots of fun problems! For one, the front brake is stuck in braked position. Eyeballing the calipers, looks like maybe just the right is gripping?

Reservoir is empty and brake handle gives no resistance. I assume the issue is in the master cylinder....

Any suggestions on how to diagnose & repair are appreciated!

 

[98lee]

Rob,

First thing to try is disconnect brake line from caliper. If brake releases, problem is with brake line or master cylinder.
If brake doesn’t release either the pistons are stuck in the caliper or the pads are rusted to the rotor. Try removing the caliper mounting bolts (NOT the bolts holding the two caliper halves together). Try knocking the caliper loose from the rotor with a wood block and hammer. Moderate taps in different directions.

Let us know what happens.

 

[rawhites]

Thanks! Released both lines, unbolted both calipers. The left came off easy. The right didn't release and took some knocking but came off....no sign on the rotor (to my eye) of the pad rusting to it.

 

[98lee]

Rob,

Remove the pads in the right caliper. See if they are stuck in the caliper. I should have had you do that earlier. If pads are stuck in caliper, it is possible the wrong pads are installed. BMW changed pad backing plate size by a millimeter or two mid production on the fronts only. If you think the pads might be binding in the caliper, let me know and I’ll get you the dimensions of the correct and wrong pads.

Use larger channel-locks and try to compress pistons back into caliper. If it doesn’t work at first, try removing bleed screw.

I’ll keep a closer eye on this thread since you’re currently working on this.

 

[98lee]

For 2v K100s and all K75s:

Front pads before 9/88 should be FA-18. Their dimensions are 55.7mm wide x 55mm tall.

Front pads after 9/88 are FA-171. Their dimensions are 57.7 mm wide x 53.7mm tall.

All rears are FA-18.

These are for the 2 piston Brembo calipers. Newer 4v bikes use 4 piston calipers.

If someone put FA-171 in a pre-9/88 caliper, it would bind. They are only .079” wider.

 

[rawhites]

Thanks, Lee! Pulled the pads and they're all identical: 55.7 x 55mm.

It *seems* like the pistons are flush in the calipers for both sides.

For best course of action....reassemble (with new pads) and refill the brake line?

 

[98lee]

Rob,

See if you can squeeze each piston further into the caliper. Then, take a block of wood that is about 1/4- 3/8” less than the distance between the pistons. Place block between pistons. Gently apply air to the brake line port (with the bleed screw installed). The air pressure should cause the pistons to advance. If both pistons advance, and you push the pistons back by hand, the pistons are not stuck. Reassemble and bleed the brakes.

If only one piston advances and can be pushed back, try rigging up something to hold the free piston in all the way. A couple of pieces of flat bar steel with a couple of bolts works well. Put a new block of wood with the 1/4- 3/8” gap between the stuck piston and the bar that is clamping the free piston. Carefully, increase the air pressure to see if you can pop the stuck piston free. If you can get it so it easily moves in and out, reassemble and bleed the brakes. If you can’t free up the piston, you will have to rebuild or replace the caliper.

When bleeding the front brakes, keep the handlebars turned all the way to the left. If you don’t, there is a spot at the hose end of the master cylinder that creates a high spot that traps air. After fully bleeding the front brakes, with the reservoir cap off and the bars fully to the left, pull in the lever and let it flick off your fingers so that the lever snaps back. This will cause micro bubbles to rise out of the bleedback hole in the reservoir. Keep doing that until no more micro bubbles appear.

I highly recommend EBC HH pads. Highly fade resistant, gentle on rotors and low dust, no squeal. You want size FA-18 for all 3 of your calipers on your ‘85.

 

[rawhites]

Amazing stuff, Lee, thank you! I'll go through all these steps and let you know how it turns out!

 

[98lee]

Rob,

Just be careful and be aware that air pressure behind the pistons can build up very large forces. When/if the piston breaks loose, the piston will slam into the wood block with great force. KEEP YOUR FINGERS OUT OF THE WAY!!! I think we already have a “Stumpy “ here on the forum so you’ll have to pick another name.

 

[rawhites]

Noted!

 

[Lendog]

Noted!

If you’re calipers have metallic pistons don’t force them in, be gentle, they should be replaced with composite pistons,
I like to use spiegler brake hoses as well,

 

[Lendog]

Rob,

Just be careful and be aware that air pressure behind the pistons can build up very large forces. When/if the piston breaks loose, the piston will slam into the wood block with great force. KEEP YOUR FINGERS OUT OF THE WAY!!! I think we already have a “Stumpy “ here on the forum so you’ll have to pick another name.

brake fluid has a warm feeling but burns in your eyes

 

[98lee]

If you’re calipers have metallic pistons don’t force them in, be gentle, they should be replaced with composite pistons,
I like to use spiegler brake hoses as well,

Lendog,

The two piston Brembos on early Bricks all had metallic pistons. Not aware who makes composite replacements.

 

[98lee]

brake fluid has a warm feeling but burns in your eyes

Lendog,

I’m so used to wearing safety glasses that I neglected to mention this precaution. Thanks.

 

[AntonLargiader]

For freeing a stuck piston I took a small chunk of metal C-channel and ground the ends down into a [ shape that just slides into the caliper. It blocks the good piston from moving and leaves room for the stuck piston to go out.

 

[rawhites]

Thanks, all! Have to travel for a few days but will make progress next week.

 

[Lendog]

Lendog,

The two piston Brembos on early Bricks all had metallic pistons. Not aware who makes composite replacements.

My Guzzi has the same Brembo calipers, I got the pistons from Harpers, If I remember right they were brembo parts

 

[rawhites]

Hey Lee - both left & right have one stuck piston and the working pistons doesn't want to go back in. Looks like I'm replacing/rebuilding both sides.
But is rebuilding that much of a savings over replacing?

 

[Lendog]

Hey Lee - both left & right have one stuck piston and the working pistons doesn't want to go back in. Looks like I'm replacing/rebuilding both sides.
But is rebuilding that much of a savings over replacing?

I’m not sure if anyone has oem calipers in stock or in rotation

 

[rawhites]

Rob,

Just be careful and be aware that air pressure behind the pistons can build up very large forces. When/if the piston breaks loose, the piston will slam into the wood block with great force. KEEP YOUR FINGERS OUT OF THE WAY!!! I think we already have a “Stumpy “ here on the forum so you’ll have to pick another name.

Hey Lee - sorry, had to step away from the project for a few weeks. It seems both right & left calipers have one stuck piston (symmetrically, which is curious), so I think I'm going to look for used replacements instead of rebuilding. Looks like there are affordable options out there. Thanks for your help and the warning...you're right that the free pistons could have taken a digit off!