85 K100 Basic - frozen front brake

[Lendog]

one other option is the FO9 calipers that have a different piston diameter, can’t remember the exact size

 

[rawhites]

Looking to finally get the new front calipers back on the bike this weekend...starting with a bone-dry reservoir & brake lines. I assume I need to take apart the master cylinder and inspect this closely before running fluid back into the system and do a full bleed process on left & right sides.
Anything I should be looking for with a MC that has been sitting dry for who knows how long? The lines seem fine on the outside.

 

[omega man]

Would you be inclined to bench bleed the master before installing?
OM

 

[rawhites]

Sounds like that makes sense...do I need a special kit to do that, can I DIY it with available tubing and an extra pair of hands?

 

[omega man]

Any time I have changed a master cylinder I have bench bled them. It was usually a line(s) from the port into the reservoir beneath the fluid level and work the piston until there were no bubbles.
This insured the internals were air free. On an older M/C it can, or may, allow a clue to the efficiency of the piston stroke via the feel of piston resistance.
OM

 

[rawhites]

Any time I have changed a master cylinder I have bench bled them. It was usually a line(s) from the port into the reservoir beneath the fluid level and work the piston until there were no bubbles.
This insured the internals were air free. On an older M/C it can, or may, allow a clue to the efficiency of the piston stroke via the feel of piston resistance.
OM

Thanks, will give it a shot this weekend.

 

[98lee]

Rob,

The master cylinder is mounted with a distinct upward tilt at the forward end of the bore. The fluid exits from the bottom of the forward end of the bore. If the handle bars are pointed straight ahead or turned to the right there is a high point at the forward end of the bore that will trap air.

If the bars are turned all the way to the left, this tip of the master cylinder bore is no longer the high point. This allows the air to continue up to the bleedback port in the master cylinder.

So basically, when bleeding the front brakes, turn the handlebars all the way to the left to prevent trapping air in the forward tip of the master cylinder.


The other issue you could have is, that if you you are manually bleeding a used master cylinder, there may be some pitting in the bore. Pitting generally occurs on the bottom of the bore, in the area forward of its normal range of travel, where moisture settles. When you manually bleed, by pumping the lever, you move the piston further than it normally travels, which runs the seals over these pits, potentially damaging the seals. That is why pressure or vacuum bleeding is preferred, where you are not moving the piston over these pits. If you must manually bleed, you can tape a 1/2” block of wood to the right grip, so the travel of the lever is limited to the normal travel, while you are bleeding.

 

[rawhites]

Thanks, Lee. I tried to get into the master cylinder and I found the actuator end (pushed by the brake lever) was covered in corrosion. I managed to dig all that out and locate/remove the c-clip. But the piston assembly won't budge....seems it's corroded in place. I don't see a way to break it free, so I assume that means I'm in the market for a new master cylinder. Sigh.

 

[98lee]

Rob,

Brake Master Cylinder 13mm - BMW Airhead, K Bike Models; 32 72 2 310 786 / BMW

Brake Master Cylinder 13mm - BMW Airhead, K Bike Models; 32 72 2 310 786 / BMW

www.euromotoelectrics.com

From BMW:

32 72 2 310 786​

CYLINDER WITH PISTON - D=13MM​

0.39​

1​

$250.00​

First time that I have seen a part that was more expensive at Euromotoelectrics than the dealer. It’s posible that BMW has had a price increase that hasn’t yet been updated on the parts fiche.

EME has some in stock. Your dealer would have to order it. Who knows how many BMW has in the country.

 

[rawhites]

Thanks, Lee!

 

[AntonLargiader]

Pretty sure that is simply a part that EME buys from BMW. BMW cracked down on Brembo for selling to the aftermarket, so it's all probably going through BMW now.

If you scrape the residue away from around the piston, you will create a free path for it to come out. Push it all the way in and then clean around the opening.

Apple Hydraulics will rebuild it for you, but I don't know that it would be cost effective. For the MCs that cost $400+, it sure would be. I will be sending a few R1100 MCs to them.

 

[98lee]

Apple Hydraulics will rebuild it for you, but I don't know that it would be cost effective. For the MCs that cost $400+, it sure would be. I will be sending a few R1100 MCs to them.

Thanks for the info Anton. That is especially interesting for the NLA rear masters. I checked their website and they resleeve those with brass. I assume that brass would not pit as easily as the aluminum bores.

 

[rawhites]

Pretty sure that is simply a part that EME buys from BMW. BMW cracked down on Brembo for selling to the aftermarket, so it's all probably going through BMW now.

If you scrape the residue away from around the piston, you will create a free path for it to come out. Push it all the way in and then clean around the opening.

Apple Hydraulics will rebuild it for you, but I don't know that it would be cost effective. For the MCs that cost $400+, it sure would be. I will be sending a few R1100 MCs to them.

Thanks, Anton. It's frozen all the way in. I worked and worked on the corrosion but seems like there's zero give/movement on it. And seems like it's a breakeven proposition to buy vs rebuild.