BMW K bikes (Bricks)

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kenrams

kenrams
active member
active member
Guys,

I recently purchased a neglected 86 K75 (44,000mls) and am working my through it. The front brakes didn't work and on taking the reservoir apart, I found a load of gunk inside and also inside the cylinder. I cleaned it up with using a piece of wire, brake cleaner, blasting air and finally an ultrasonic bath. It is now spotless and both holes are open as can be seen in the pictures below. Also, the piston, seals, and cylinder bore all look in excellent condition. 

So why the post? When I fill the reservoir with fluid I get no output to the brake line when I pump the brake lever! There is no brake fluid escaping anywhere either. This does not make sense to me and can only guess that the rubber piston seals are not as good as they look. Do I need to buy a rebuild kit (new piston and seals) ? 

Cheers
Ken  


[solved]Front brake reservoir/cylinder issue Pc070110
[solved]Front brake reservoir/cylinder issue Pc070112
[solved]Front brake reservoir/cylinder issue Pc070111


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

MartinW

MartinW
Life time member
Life time member
Normally if you fill the master cylinder with fluid and block the outlet you should get pressure when the lever is pumped. WARNING don't get brake fluid on your paintwork. Even with slightly scored cups you should get some pressure build up. Picture shows cups correctly orientated you haven't changed them since I presume??
Regards Martin.


__________________________________________________
K75s Hybrid
    

Dai

Dai
Life time member
Life time member
My guess is the seals are duff, but you might also have an air bubble in the line that's taking the p!ss. A reasonable (but not foolproof) check is to push the caliper pistons hard home, then try bleeding. What that does is to reduce the amount of air space in the hydraulic system.


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

Woodie

Woodie
Life time member
Life time member
Have you removed the lines at the calipers?  I would test for flow at every joint past the reservoir until you locate where the blockage might be.  Bleeding the front brakes is a pain but be patient and eventually you will get everything worked out.


__________________________________________________
1985 K100RT
52667
"Keep your stick on the ice.  We're all in this together."  Red Green
    

kenrams

kenrams
active member
active member
Thanks for the replies, Here’s what happened. I tried the usual bleeding procedure which I have done many times and no fluid came out of the nipple. I removed the brake line at the reservoir and still nothing came out when I pumped the brake lever. I removed the reservoir and that’s when I found all the crud. Cleaned it as described and put it back together and still get the same result. I can only think it’s the seals on the piston but they “appear” fine. Looks like I’ll just have to buy the rebuild kit today.

    

Laitch

Laitch
Life time member
Life time member
@kenrams wrote:I tried the usual bleeding procedure which I have done many times and no fluid came out of the nipple. I removed the brake line at the reservoir and still nothing came out when I pumped the brake lever. I removed the reservoir and that’s when I found all the crud. 
Have you checked the brake lines for crud?

The first part of the video involves the master cylinder; the second part—at 1:29—involves one of the brake lines.


__________________________________________________
1995 K75 81,000 miles
[solved]Front brake reservoir/cylinder issue Usa-lo10
    

kenrams

kenrams
active member
active member
At this point in the procedure, the lines are disconnected and I am getting no fluid out of the bottom of the brake cylinder when I pump the lever.


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Are you absolutely sure both holes are open?  I only saw the large one in the photos. 

Do any air bubbles rise into the reservoir when you squeeze the lever or tap the reservoir?

With no piston in the master cylinder, will the fluid flow down from the reservoir? 

Is any fluid leaking from the lever end of the piston?

Did you lubricate the piston seals with brake fluid before you put it together?  I soak mine in a small pan of fluid for a minute or so to make sure there are no dry spots between the seals and the piston.  Some guys even put some fluid in the bore when they assemble the master cylinder.


__________________________________________________
Present:
1994 K75RT
1991 K100RS
1988 K100RS SE

Past:
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

9Back to top Go down   [solved]Front brake reservoir/cylinder issue Empty Problem Resolved on Sat Dec 08, 2018 3:30 pm

kenrams

kenrams
active member
active member
I took my K100's reservoir apart and removed the known good piston (and seals). I fitted it into the K75 and after two pumps of the lever the brakes came back to life. 

So the lesson is that although the old piston and seals looked "good" they actually were not. 

Thanks for the input guys! I am sure I will be back with questions as I work my way further through the bike this winter


__________________________________________________
1981 R100 cafe- 121,000 miles
1985 K100 cafe - 55,000 miles
1987 K75C - 44,000 miles
    

Dai

Dai
Life time member
Life time member
Very Happy


__________________________________________________
1983 K100 upgraded to K100RS spec, 1987 K100RT
Others...
1978 Moto Guzzi 850-T3, 1979 Moto Guzzi 850-T3 California,1993 Moto Guzzi 1100ie California
2020 Royal Enfield Bullet 500
    

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