are K75 front brake master cylinder pistons interchangeable?

I have used the search function and did some browsing, so please forgive me if the answer to the following question is in the forum and I didn't find it.

Do all the K75's use the same front master cylinder piston? The A&S fiche has one listed for the K75, but not the K75C/RT/S. I suspect they do use the same piston, but thought I'd ask you experts because I noticed the "cylinder with piston" part number is the same for K75C/RT/S and different for the K75.

The reason for asking is that I'm getting some weeping from the piston area and want to get it taken care of. I was hoping just the seals are available but apparently not, unless one of you happens to know of an alternative for them.

Really appreciate being able to rely on you guys for advice and help.

Thanks!

The ABS bikes use a different piston (and cylinder housing) than the non-ABS bikes, even though they are both 13mm.

Which do you have?

98lee said:

The ABS bikes use a different piston (and cylinder housing) than the non-ABS bikes, even though they are both 13mm.

Which do you have?

Click to expand...

Doh!!! I have a non-abs bike. The fiche for C/RT/S are all labeled in all caps "ANTI BLOCK SYSTEM". The fiche for the standard has no such label, so that's the one for me. Sorry to have bothered you!

Here's the one you want: 32 72 2 302 356 PISTON - D=13MM $79.57



BMW only carries the piston (with seals) for the non-ABS K75s (all models).

Beemerboneyard only carries the piston (with seals) for the ABS K75s (all models).


Inspect the bottom half of the bore for any pitting before putting the new piston in.


A flapper hone can be made from a 1/4" wooden dowel and some very fine sand paper to clean up MINOR inperfections in the bore. Put a 1" deep slit in the end of the dowel. Put a 1" X a few inches of sand paper, folded in half so the grit faces out on both sides so that it' 1" X half as long. Slide the paper in the dowel slot then twist dowel clockwise while holding the ends of the paper, wrapping the paper around the dowel.
Put the dowel in your drill motor and LIGHTY hone the cylinder. Use only to remove crud and MINOR inperfections. If the bore is pitted, the amount of material required to be removed will trash the cylinder and cause BRAKE FAILURE! A new cylinder is required if there is more than VERY, VERY minor pitting.

Thanks for all the help, Lee. I have time to disassemble and clean before deciding what I need to order. I'll definitely adhere to your advice. Not going to cut corners when it comes to brakes.

Definitely check for pits in the cylinder before you buy the piston. The new cylinder comes with a piston, you don't want to buy it twice. I had to replace my whole assembly, but I cannot complain. 25 years was a long time for a break cylinder.