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Old 08-20-2014, 08:19 AM   #1
birds OP
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Can't Diagnose Brake Issue

Hi All,

My buddy is having a brake issue that we are trying to fix for our trip in a week and a half.

I've been scouring this forum and the internet, but I think I need something a little more interactive or specific.

Symptom: When the brake lever is pressed, It is firm at first, but as it is held down, it softens and drops to the bar. Pressing it again returns pressure, but then it drops again.

Some things I've tried:
- Someone told him it was the brake lines, Which are now new stainless steel lines.
- I bled the brakes for a while after replacing the lines. I ran a decent amount through at the end with no bubbles coming out (~10 reservoirs full).
- I tried tapping the lines, calipers, and cylinder while pressing the lever and not, and while bleeding the brakes.
- I left the lever zip-tied down overnight, though I doubt this would do anything because it doesn't feel like there's any pressure there after it's held down for a few seconds.
- I very slowly pressed and released the lever 30 times, looking into the master cylinder. Not a single air bubble came out of the master cylinder.

Other:
- We changed the brake pads and cleaned up the calipers. This is after it started happening. There was a little bit of white corroded aluminum powder under the boot that covers the pistons. I didn't think much of it since it matched the corroded aluminum found elsewhere on the caliper. The pistons looked clean.

Anything that needs to be ordered needs to be ordered now. It's a 81 CB750C, not one of the really popular models for brake parts. '80 and '82 seem to have different systems. I'm thinking it's the master cylinder. I suggested getting a rebuild for that and also the seals for the calipers just in case.

Am I on the right track, or am I missing something? Is it just tenacious air bubbles? I don't think so, because the lever is firm when first pressed. It just gets softer and sinks, as if fluid is allowed to flow back up through the master cylinder. However, I'm no expert, and have only been doing this for myself.

Is there anything else I should try? unfortunately I didn't read about clamping the rubber lines to see where the issue was. The new SS lines are not for clamping. We're running out of time for the usual trial-and-error, so any advice would be greatly appreciated.
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Old 08-20-2014, 08:34 AM   #2
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Sounds like the piston is weeping fluid by. Order a rebuild kit and install. You've essentially just described a slow hydraulic leak. If it was the caliper,it would more than likely be making the pads wet. Your master cylinder will simply bleed/leak by into the reservoir again. Check for leaks,order as needed. If no leaks are external,I'd rebuild the master cylinder.


http://www.ebay.com/itm/like/231162665296?lpid=82

This is out of an older DR650. Works the same way. When it's pushed in,it forces fluid into the line and since it wont take pressure,ends up pushing the piston(s) out. If the inner seal (by spring) is worn,tore,cracked,etc,it'll let fluid weep by. Depending on the damage,it can be firm at first then lose pressure,or never have any to begin with.


Edit: damned auto correct

Danjal screwed with this post 08-20-2014 at 09:08 AM
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Old 08-20-2014, 08:46 AM   #3
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+1 on the MC.
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Old 08-20-2014, 08:57 AM   #4
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Awesome, thanks a lot guys!

One step closer to not crashing in a fiery wreck
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Old 08-20-2014, 09:04 AM   #5
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Also check your master cylinder body. The inside should be smooth and clean. Any pits,scratches,or debris will wear away those rubber seals of poke holes in them.

It's a pretty easy fix too. Remove your lever. The notch to the left in the photo holds a retaining ring. Remove it and it all comes out usually. If not a pick gets it,just don't scrape the mc body. Check it with a light for damage/debris. Reinstall,refill reservoir,tap on res to knock air out,bleed if needed,done.

Oh, don't look at the piston up close when removing the snap ring. I've seen a few come out pretty fast.

Danjal screwed with this post 08-20-2014 at 09:27 AM
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Old 08-20-2014, 10:09 AM   #6
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There are two holes/passageways at the bottom of the master resivoir. make sure that both are clear and not clogged.
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Old 08-21-2014, 05:39 AM   #7
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all of the above.

it's bleeding back past the rubble cups. replace them and check the MC for pitting,
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Old 08-21-2014, 01:34 PM   #8
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Quote:
Originally Posted by Switchblade315 View Post
all of the above.

it's bleeding back past the rubble cups. replace them and check the MC for pitting,
IF the bore of the MC is pitted, I've had good luck recently refinishing the bore on my Magura Clutch master cylinder using a FlexHone AlOx, 240 grit hone. It was leaking bad as the anodized surface of the bore had flaked and/or pitted.
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Old Yesterday, 10:15 AM   #9
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The passageways looked clean from above, but I will make sure to clean them when I disassemble.

A buddy is bringing by an ultrasonic cleaner for me to use tomorrow. Is it worth cleaning the cylinder housing in that? If so, what sort of solution should I use?

I ordered the hone for the cylinder size as well. That might get interesting, since it won't come in until mid next week.
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Old Yesterday, 10:48 AM   #10
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You don't need to do all of that,just check the housing and go from there. Is it clean and smooth? Same for the passages. If so install the kit and ride. What did your old seals look like? Old, cracked,worn,etc? That should be your biggest indicator of it being the mc seals. Push on the edges and inspect. Sometimes the holes and cracks blend in well or are very tiny. Back light helps too. Normally the cracks are inside by the aluminum piston or on the very outside.

If the mc is in good shape, don't worry about conditioning it. I've never had to yet.

A very bright inspection light or carb jet cleaner (or wire/paperclip) helps with cleaning those small holes. 99% of the time if the res and mc chamber are clean,nothing will be in the passages.

The main reason is to prevent further seal damage and make sure the job is done right the first time. Honing/polishing a mc isn't normal, but it should be checked like other components in any other job. Pads go bad- check the rotor thickness also. One of those deals where related bits are checked and replaced/repaired as needed.It's good form to do so and can sometimes save time and money by preventing the reasons for wear and breakage in the first place. We also include these things and others so folks here gain a working knowledge of how things work. Once you understand the system and it's parts,it's easier to diagnose issues, repair them,and fix the root causes. I apologize if the overload of info confuses you. It's also MUCH easier to do these things in person. Many questions,items,and explanations go unsaid on the net because the task isn't directly at hand.

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Old Today, 06:04 AM   #11
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Quote:
Originally Posted by Danjal View Post
I apologize if the overload of info confuses you.
No, I get it
I would just rather have all my bases covered before I run out of time to order what I need. If that means spending $15 on a 16mm hone that I'll never use, so be it.
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