1975 BMW R75/6: Rebuilding Master Cylinder & Caliper

This bike uses a single ATE 38 mm caliper (as in the size of the piston inside the caliper, not the size of the caliper shell). In 1976 the ATE caliper was changed to a 40 mm model to improve braking. There was also a change to the master cylinder mounting system resulting in a single hole in the brake fluid reservoir that mounts on top of the master cylinder. Other than these small changes, the work I did applies to all the /5, /6 and /7 models of the ATE caliper and under-the-tank master cylinder.

You can read about how to do this work here:

I had occasion to work on both my 1975 R75/6 and someone else’s 1976 R75/6 at the same time, so there are some pictures of the markings on the 40 mm caliper and the different mounting systems (two hole and one hole) of the brake fluid reservoir.

Some pictures from this write-up are below.

Master Cylinder & Fluid Reservoir Mount Under Gas Tank

Master Cylinder & Fluid Reservoir Mount Under Gas Tank. Note the Evidence of Brake Fluid Leak.

Earlier Style Master Cylinder Fluid Reservoir-Two Holes With Mounting Bracket

Earlier Style Master Cylinder Fluid Reservoir-Two Holes With Mounting Bracket

Later Style Master Cylinder Reservoir-Single Hole With Threaded Sleeve

Later Style Master Cylinder Reservoir-Single Hole With Threaded Sleeve

Earler Two Hole Master Cylinder

Earler Two Hole Master Cylinder

Master Cylinder Piston Assembly Removed

Master Cylinder Piston Assembly Removed

Removing Top Hat From Master Cylinder Piston

Removing Top Hat From Master Cylinder Piston

Master Cylinder Rebuild Kit Parts

Master Cylinder Rebuild Kit Parts

New Master Cylinder Piston with New Rubber Seals Installed

New Master Cylinder Piston with New Rubber Seals Installed

Master Cylinder Installed on Frame Tube

Master Cylinder Installed on Frame Tube

R75/6 Caliper Seal Kit - Source: MAX BMW

R75/6 Caliper Seal Kit – Source: MAX BMW

Caliper Showing Piston and Dust Seal

Front Face of Piston with Center Hole Surrounded by Rubber Dust Seal

A Blast of Compressed Air Removes Piston

A Blast of Compressed Air Removes Piston

Dust Seal Installed in Large Groove in Piston

Dust Seal Installed in Large Groove in Piston

Pulling O-ring On Eccentric Pin With Pick

Pulling O-ring On Eccentric Pin With Pick

Movable Pad with Flat Face of Pad Pointing to the Front

Movable Pad with Flat Face of Pad Pointing to the Front

Fixed Pad Secure with Wire Clip

Fixed Pad Secure with Wire Clip

Caliper With Painted Metal Line Installed

Caliper With Painted Metal Line Installed

5 thoughts on “1975 BMW R75/6: Rebuilding Master Cylinder & Caliper

    • Britt,

      You’re welcome. And, I haven’t made the decision yet on the the tank bra on the 1977 R100RS I picked up. If I do get rid of it I’ll let you know.

      Best.
      Brook.

  1. Piston on my 1976 R75/6 is completely frozen. Soaked, air blast, hydraulic press,
    nothing works. Any lead on where to find a used caliper?

    • Hi Dexter,

      I’m sorry to hear this one is really rusted tight.

      eBay is a good place to find these. I’ve had good luck at Duncan’s Beemers who has an eBay store
      [http://stores.ebay.com/Duncans-Beemers]

      Larry Stonestreet at Stoner’s Beemer Stash is also a source for used parts. He does not have a web site to my knowledge.
      [[email protected]; 951-813-1704].

      And a general ebay search should also yield some results.

      Best of success on overcoming this.

      Best.
      Brook.

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