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Topic-icon R100RS, R90S rear brake master cylinder

  • mikethebike
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1 month 2 weeks ago #6223 by mikethebike
R100RS, R90S rear brake master cylinder was created by mikethebike
I have an R90S with Morrison Mags put on originally at the dealership as an option. Originally they used Hurst Airheart hydraulics which are made of Unobtaniun.
I have switched the master cylinder and brake caliper for the same as installed on the R100’s. These are made by Brembo some with BMW partnumbers.
I got the master cyl and caliper from flea bay. I have new internals which are all Brembo parts.
I had a hard job driving the piston in the master cylinder out as it had obviously been dry for a long time.
But with a bore cleanup the new pistons works smoothly. However I don’t know if the outer seal is functioning correctly.
It is assembled correctly but it moves in and out as I pump the lever. It does not appear to stay down. I thought the ring around the outer seal was supposed to be a friction fit in the outer bore.
Any guidance would be appreciated.
Michael

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2 weeks 1 day ago #6437 by 13807
Replied by 13807 on topic R100RS, R90S rear brake master cylinder
Have you tried bench bleeding the master cylinder. It is a decent functionality check. The you tubes have several videos.

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2 weeks 1 day ago - 2 weeks 1 day ago #6440 by Wobbly
Replied by Wobbly on topic R100RS, R90S rear brake master cylinder

mikethebike wrote: I had a hard job driving the piston in the master cylinder out as it had obviously been dry for a long time. But with a bore cleanup the new pistons works smoothly. However I don’t know if the outer seal is functioning correctly. It is assembled correctly but it moves in and out as I pump the lever.

Michael -
Sorry but your description is not very clear.

There are 2 seals on the piston. The inner creates the braking pressure, the out keeps the unit from leaking. The piston is supposed to move in and out as you pump the lever. An internal spring forces the piston to return to the "rest" position. If it didn't, then the brakes wouldn't release when you let off the brake lever.

mikethebike wrote: It is assembled correctly but it moves in and out as I pump the lever. It does not appear to stay down.

Neither the rear brake lever nor the m/c are supposed to stay anywhere. When the pressure on the rear brake lever is released, then the piston and the level return to the "rest" position.

mikethebike wrote: I thought the ring around the outer seal was supposed to be a friction fit in the outer bore.

You just enlarged the bore by cleaning it. Whatever friction there was is now gone. What keeps all the internal parts in place is an internal circlip at the mouth of the cylinder. The various actions of the brake fluid mean that this clip often corrodes badly and/or breaks during disassembly, and cannot be reused. Any good hardware store can supply a replacement.

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 2 weeks 1 day ago by Wobbly.

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2 weeks 8 hours ago #6444 by mikethebike
Replied by mikethebike on topic R100RS, R90S rear brake master cylinder
Hi Wobbly
I couldn't find any sign of a groove in the end of the M/C. The piston and return spring all function perfectly. But the outer seal and ring just moved in and out as the piston did.
So I found a perfect sized compression spring that fitted snug on the piston and seated exactly on the separate seal ring. The spring is kept in place by the lever arm and hidden beneath the boot. Testing with out the boot showed that the outer seal did not move in and out anymore and I dont think there is any hydraulic pressure between the two inner piston rings and the outer seal.
I'm finishing the bending to get the line into the M/C today and will bleed te system and see if I get fluid puking out anywhere..
Michael

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