BMW K bikes (Bricks)

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C_Mick

C_Mick
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While the 75 was stored in my garage the reservoir of the rear master cylinder disintegrated leaking brake fluid all over the place and giving the RHS footplate a severe case of Japanese frostbite. The replacement m/c reservoir should reach me tomorrow, but I wanted to check before filling up with brake fluid that the caliper pistons were working OK. Curiously the pistons were in the pushed fully into the caliper position and I was trying to think of a way to see if they would move freely. I'd read in a post on this forum that it works to use an air line to drive a piston out of its bore in the caliper. That prompted me to think of using my car tyre inflater which is is driven via a 12V power socket.

Checking for stuck / frozen caliper pistons K75S_testing-caliper-piston

It worked very well and on a setting of approx 3.5bar the pump applied enough force to the disc pads for me to be satisfied that one or both pistons are not frozen. I had already checked to see if any brake fluid had leaked past the caliper seals, but fortunately it hadn't.

Chris


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Two wheels good...Four wheels bad...
    

92KK 84WW Olaf

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Having the caliper pistons pushed fully home means they should be ok as they would have been kept lubricated by the fluid and hopefully protected from corrosion. If you get a problem you are more likely to find the brakes binding so just keep an eye on the, if they bind they will get hot.

I had a similar problem with the rear master cylinder but a seal kit was used to repair rather than replace it. It has lasted fine.


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1992 K100LT 0193214 Bertha Blue 101,000 miles
1984 K100RT 0022575 Brutus Baja Red 578 bought 36,000 now 89,150 miles
1997 K1100LT 0188024 Wotan Mystic Red 689 58,645 now 106,950 miles Deceased.
1983 K100RS 0011171 Fricka 606 Alaska Blue 29,495 miles Damn K Pox Its a Bat outta Hell Now 37,190 miles. 
1996 K1100LT 0233004 Lohengrin Mystic Red 38,000 miles currently 42,640 miles.

Past:
1968 Yamaha 80 YG1
1971 Yamaha 125 YAS-1
1968 Honda 125 SS
1970 Honda CD 175
1973 Honda CB500-4
Honda CX 500
    

C_Mick

C_Mick
Silver member
Silver member
92KK K100LT 193214 wrote: ...I had a similar problem with the rear master cylinder but a seal kit was used to repair rather than replace it. It has lasted fine.

I dismantled the rear m/c and there was some corrosion, but the seals looked OK and the corrosion was above the normal travel movement of the piston seal, so I decided to polish the bore instead of buying a seal kit. I should know today or tomorrow whether I made the right decision.

Thanks for your comments. Chris


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Two wheels good...Four wheels bad...
    

charlie99

charlie99
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good stuff chris ...


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cheezy grin whilst riding, kinda bloke ....oh the joy !!!! ...... ( brick aviator )

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%OChecking for stuck / frozen caliper pistons Au-log10

'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
    

C_Mick

C_Mick
Silver member
Silver member
Just to round off the saga of the rear m/c: the replacement reservoir arrived on Monday and I fitted it and filled up with brake fluid yesterday. Haven't been able to road test the brake yet, but the brake line works fine. Regarding the screw adjuster at the front of the m/c, how do you judge how far to screw it in? Should there be a tiny bit of free play before the screw rod engages the 'pusher' cup that bears on the end of the piston? Thanks in advance.

Chris


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Two wheels good...Four wheels bad...
    

Inge K.

Inge K.
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@C_Mick wrote:Should there be a tiny bit of free play before the screw rod engages the 'pusher' cup that bears on the end of the piston?
That`s correct...and this small amount of free play is quite important....to be shure that the piston returns fully to its rest position....every time.

To avoid a binding rear brake, which can be hotter than h...


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Inge K.
K100RS -86. (first owner), K1100LTSE -94.
    

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