Author Topic: Front brakes, no pressure after rebuild 86 k75c  (Read 488 times)

Offline Gorgerunner23

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Front brakes, no pressure after rebuild 86 k75c
« on: August 12, 2020, 09:53:50 AM »
Hi guys. I just rebuilt the front master cylinder on my 1986 k75c non ABS.  I also replaced the brake line that goes from the master cylinder to the triple tree as Iíve upgraded to strap ons and the stock line was not long enough. I bled all the air out of the system at each caliper. No more air bubbles come out at all. I still have no pressure at the brake lever and I can see the brake pads do not move at all. There are no leaks in the system anywhere. Iíve tried using a zip tie to keep the lever squeezed over night to try and get any air out the master cylinder that may have been trapped. Iíve left the bars turned to the left over night. I feel like Iíve tried everything I can think of. Possibly I can bleed the master without the line attached just to ensure itís not air locked. I would love to hear your thoughts. Thank you.
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Offline Laitch

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #1 on: August 12, 2020, 10:26:24 AM »
Possibly I can bleed the master without the line attached just to ensure itís not air locked.
You should verify that the master cylinder is working by doing that. There is a leak or blockage somewhere. Have you verified that both ports of the piston are not blocked?


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Offline Gorgerunner23

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #2 on: August 12, 2020, 11:46:10 AM »
I have not specifically verified that. I did blow compressed air through both holes while cleaning the cylinder. I will re address those holes again. Thank you.
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Offline alabrew

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #3 on: August 12, 2020, 02:12:52 PM »
When I replaced the front lines on the '85, I ended up having to push fluid from the caliper to get all the air out and and master pushing again.
  • Birmingham, Alabama
  • 1985 K100, 1991 K100RS
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Offline Gorgerunner23

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #4 on: August 12, 2020, 10:34:53 PM »
UPDATE! Reverse bled the system with a syringe. All is good now. The master cylinder was air locked. Thanks everyone.
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Offline President-elect The Mighty Gryphon

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #5 on: August 12, 2020, 11:07:23 PM »
Can you explain what you did with this reverse bleed?  Front Master Cylinder problems are not uncommon, and any methods that work are useful.
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Offline Gorgerunner23

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #6 on: August 13, 2020, 06:13:45 AM »
Absolutely. I used a large syringe from tractor supply. Put a length of hose on the end of it connected to the bleeder on the caliper. Make sure there are no air bubbles in the syringe or the hose. Make sure the cap is removed from the master cylinder and also, itís a good idea to make sure to remove most of the fluid from reservoir before using this method. You can use the syringe to drain the reservoir. Open the bleeder valve.  Slowly inject the fluid through the caliper. Watch the mater cylinder reservoir too see when the bubbles stop coming out. Tighten your bleeder screw. Repeat on the other caliper. Hope this may help anyone who runs into a similar problem! 
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Offline President-elect The Mighty Gryphon

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #7 on: August 13, 2020, 08:12:26 AM »
Nice!!!  Added to the memory banks.
  • In my garage in Marilla, NY
  • '94 K75RT Mystic Red, '91K100RS White/Blue, '89 K100RS Special edition White/Blue
Current:
'94 K75RT "Ilsa, She Wolf of the SS"
'91 K100RS "Moby Brick Too
'89 K100RS SE

Offline natalena

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #8 on: August 13, 2020, 09:30:26 AM »
Absolutely. I used a large syringe from tractor supply. Put a length of hose on the end of it connected to the bleeder on the caliper. Make sure there are no air bubbles in the syringe or the hose. Make sure the cap is removed from the master cylinder and also, itís a good idea to make sure to remove most of the fluid from reservoir before using this method. You can use the syringe to drain the reservoir. Open the bleeder valve.  Slowly inject the fluid through the caliper. Watch the mater cylinder reservoir too see when the bubbles stop coming out. Tighten your bleeder screw. Repeat on the other caliper. Hope this may help anyone who runs into a similar problem!

Great info GR23.
May I add, when I did this, it was with the brick on it's side stand and handlebars turned to the left lock stop. Tapping the junctions with a screwdriver handle helps give you confidence that air isn't hiding along the line.
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Offline alabrew

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #9 on: August 14, 2020, 06:33:08 PM »
Apparently, that is how they fill at the factory, at least with the K1200LT per jzeiler - Wrencher Extraordinaire, at bmwlt.com

  • Birmingham, Alabama
  • 1985 K100, 1991 K100RS
Also:
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Offline stokester

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Re: Front brakes, no pressure after rebuild 86 k75c
« Reply #10 on: August 15, 2020, 07:36:00 PM »
UPDATE! Reverse bled the system with a syringe. All is good now. The master cylinder was air locked. Thanks everyone.
Good info but I would only recommend a reverse bleed after the system has been flushed.  Running contaminated fluid through the MC is not a good idea.
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