BMW K bikes (Bricks)


You are not connected. Please login or register

View previous topic View next topic Go down  Message [Page 1 of 1]


1Back to top Go down   Rear master cylinder Empty Rear master cylinder Sun Dec 08, 2013 5:10 am

Guest

avatar
Guest
I have a rear brake issue I don't understand. The system bleeds ok and I have pressure on the pedal and the brakes work, but hold that pedal down for a while and it sinks to the end of travel and the brakes fail. No good at all at traffic lights on a hill Smile

I thought the caliper was leaking and put a seal kit into it today as well as a new stainless hose, the original had cracks appearing, and the issue is still there. I don't seem to be loosing any fluid either, if anything, it sort of looks like the level in the reservoir goes up a little as the pedal goes to full travel and I can't find any leaks anywhere.

The only thing left is the rear master but they don't make a kit for it so a new one will be around $130 delivered and I figured I'd try pulling the master apart for a look see before ordering a new one.
 
I just wanted to check that there isn't something else to look at before spending money Smile.

    

2Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 5:36 am

charlie99

charlie99
VIP
VIP
commonly the bores become pitted (water contamination ?)

I have successfully pulled the piston and cleaned up the bore with a makeshift hone (was on the front master ...and a few others )

get some 10 mm dowel and cut a cross in the end ...deep enough to hold some wet and dry  400 or higher grit ....of about 15mm in width and maybe 40-50 mm long ,,wrap the wet and dry around the dowel in a layered fashion with one end through the cut in the dowel

then progressively work the cylinder with the hone with a cordless drill ...easy does it !

a new piston seal would probably be best option if they were available because the "edge " disappears real quick when travelling along a pitted bore


I have got some from some smaller carparts dealers before  mostly pbr because they were pretty common ...but worth asking around

 
 
looks as though you have nothing to loose but for the trying huh ?


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

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
1987 k100rs ########   "Red"  - (red sports rs TWB style )
1989 K100rt #009637   "Black Betty"  (naked rt ala Nigel , now sporting an rs main fairing )
    

3Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 5:55 am

Guest

avatar
Guest
Cheers Charlie, yes it's definitely the master cylinder. I pulled it off the bike and the push rod and boot comes away from the piston easily and when I drive the piston in/out I see fluid seeping back out around it. 

The piston is held in there with a circlip which I can't get out. I've managed to bend it a little but it don't want to come out. I might see if I can get/invent a circlip tool before I destroy it. The one I have works in the wrong direction.

    

4Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 6:00 am

charlie99

charlie99
VIP
VIP
real quick run to supercheap perhaps and get the right ones before you destroy the whole thing


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

'86 K100 RT..#0090401 ..."Gerty" ( Gertrude Von Clickandshift ) --------O%O
'86 k100 rs.. #######..  "Fred " (f(rame) red ) ( Fredrick leichtundschnell ) - -
bits and pieces from many kind friends across the k100 world ...with many thanks ..
1987 k100rs ########   "Red"  - (red sports rs TWB style )
1989 K100rt #009637   "Black Betty"  (naked rt ala Nigel , now sporting an rs main fairing )
    

5Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 6:23 am

Guest

avatar
Guest
Can't even get onto their website, imagine the crowds at the shop today Mad 

Found some tiny pliers and out she came. There is a plastic tube at the bottom below the bore with an o'ring on top of it. I think that one is leaking a bit as well. Looks a lot like a 1/2" one off the garden hoses. I'll try a cleanup job on the bore and look around for another o'ring.  Cheers.

Rear master cylinder Rear_master

    

6Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 9:43 am

MikeP

MikeP
Life time member
Life time member
If the pressure falls and no fluid is lost, it must be that the pressure loss is due to fluid passing the piston. If the O-ring fails, as its purpose is to keep fluid inside the chamber, I'd expect to see a leak.

Damage to the piston bore is possible but much more likely is the rubber seal on the piston itself allowing fluid to seep past while the piston is depressed for any length of time.


__________________________________________________
1992 K1
1993 K1100R (used to be an LT)
    

7Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Sun Dec 08, 2013 12:32 pm

Guest

avatar
Guest
Yep, the piston seal is between the piston and the spring in that photo. Fluid is passing that, being stopped by the 1/2" o'ring and going up the tube to reservoir, hence no outside leaks. I couldn't fix it, ordered a new one.

    

8Back to top Go down   Rear master cylinder Empty Same problem (nearly) Mon Jan 20, 2014 4:16 pm

Banditrob

Banditrob
active member
active member
My K100Lt has been sitting in the garage for a couple of months, and I noticed the rear brake lever travel had increased by about a inch and half before activating the brake.

The fluid level was normal and I couldn't find any leaks, so I assumed air must be in the system.

I purchased a one way valve little bleeder kit and bottle of dot 4 fluid and started to bleed the system.

At first a small amount of fluid appeared in the tube about an inch long and then some air bubbles appeared, as I continued to pump the pedal nothing else came out and the reservoir level stayed the same.

Now I have no brake activation at all.

Could this be a master cylinder fault.


__________________________________________________
K100LT, Suzuki Bandit 1200 Mk 1 and Honda C90
    

9Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Tue Jan 21, 2014 12:33 am

Rick G

Rick G
admin
admin
sounds to me like the master cylinder piston has stuck at the bottom of the bore.  Replacing the seals and honing the MC is a good place to start and it will probably finish there as well. Tip there is a small socket head cap screw (allen head) on the back of the MC which needs to be removed to allow the piston to come out.


__________________________________________________
"Man sacrifices his health in order to make money.
Then he sacrifices money to recuperate his health.
And then he is so anxious about the future that he does not enjoy the present; the result being that he does not live in the present or the future; he lives as if he is never going to die, and then dies having never really lived."   Dalai Lama


Bikes 1999 K1100 LT with a Big Block 1200
    

10Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Tue Jan 21, 2014 3:17 am

Guest

avatar
Guest
When I replaced my master cylinder before xmas, it did the same thing when I went to bleed it. No brakes at all. I tried pumping fluid in from the calipers but in the end I had to pour some fluid in the top of the master cylinder where the brake line attaches. I guess that primed the line

    

11Back to top Go down   Rear master cylinder Empty Re: Rear master cylinder Tue Jan 21, 2014 3:31 pm

Banditrob

Banditrob
active member
active member
Thanks for the information, will inspect closer.


__________________________________________________
K100LT, Suzuki Bandit 1200 Mk 1 and Honda C90
    

Sponsored content


    

View previous topic View next topic Back to top  Message [Page 1 of 1]

Permissions in this forum:
You cannot reply to topics in this forum