master cylinder will not allow fluid back into reservoir

[SIR REAL ED]

2007 Nevada 750.
 
The bike has been dropped a couple times by the previous owner due to the front brake locking up. 
The master cylinder piston moves forward and back when the brake lever is pulled and released.  But the fluid does not flow back into the reservoir when the brake lever is released and the pistons are pried back.

All the pistons in the caliper move freely but, as one would expect, pushing one back forces another out.  Disconnecting line from master cylinder allows all the pistons in the caliper to be pushed back.  After remove the white plastic plate from the bottom of the master cylinder reservoir, I see only one hole in the bottom of the reservoir.  Putting compressed air on the outlet of the master cylinder confirms that there is no flow back into the reservoir.   

I have not pulled out the piston and seals but I do not see any ports I can clean to fix the problem.

Any thoughts?

Are master cylinder rebuild kits available?

Thanks in advance for your help.

[Kiwi_Roy]

Perhaps the master cylinder is out of adjustment and the piston is not moving back far enough to uncover the return hole.
As its been dropped it may be a little bent to put it out of adjustment, if you take the top off you should be able to see the hole.

[SIR REAL ED]

Perhaps the master cylinder is out of adjustment and the piston is not moving back far enough to uncover the return hole.
As its been dropped it may be a little bent to put it out of adjustment, if you take the top off you should be able to see the hole.

I think you may be on to something.  I disconnected the brake line and reached into the banjo bolt hole with a punch and and the piston seems to have full movement when I pull in and release the brake lever.  Even if I try to lightly tap the piston in a little bit, it does not move back.  IIRC there is not adjusting screw on the brake lever, maybe I did not look at it close enough.  BUT.... the brake lever is ever so slightly bent, so it is possible that the brake lever is preventing the piston from moving back just enough (1 or 2 mm?) to not allow the fluid to vent back into the reservoir.

[wirespokes]

That would be my guess - bent lever preventing full piston travel.

[Wayne Orwig]

Be aware, if the fluid heats up and expands, it can press on the brake since it is unable to return to the reservoir. That can make the brake drag, heat more, expand more, drag more. Pretty quickly it can lock up the brake. A big safety issue.
Get it fixed.

[Ryan]

2007 Nevada 750.
 
The bike has been dropped a couple times by the previous owner due to the front brake locking up. 

Perhaps this issue pre-dates the bike being dropped?  If taking the brake lever off doesn't allow the fluid to return, then you have deeper issues. What does a new master cylinder cost?  Might be faster and cheaper in the long run to just go new and be sure.

[Chuck in Indiana]

Wayne explained it perfectly..

[Kiwi_Roy]

Also if the bike has been sitting for several years the brake fluid can take on the appearance of sugar crystals and plug holes through the metal fittings
I once bought a Goldwing with only 350 km on the oddo, the lines on it were packed solid with what looked like crystallized sugar.

[SIR REAL ED]

I appreciate all the knowledgable replies.  You guys would make good detectives.  Thank you all very much.

I am about 99% certain the piston is traveling its full stroke. 

The bike has been sitting a lot and I'm pretty sure brake fluid degradation/contamination is the problem.  Brake fluid did not look as bad as some I have seen though.  I'm just surprised I can't see the vent hole.  One hole in the bottom of the reservoir, and a second location that looks like it should be a hole, but it is so solid that I can't tell whether it is plugged or just the way the reservoir is made.  Even light tapping with a punch, and pushing a small drill into this location and twisting by hand does not "clean out the hole."   

According to the previous owner, "his friend" crashed the bike when the front brake locked up, and went down in on a gravel road.  The previous owner warned him about the brakes locking up when the bike heated up, which is why he only took it for a short ride.  Apparently, not short enough.

Too bad, the bike was pristine.  Now I am going to replace the right muffler, brake pedal, and right turn signal, and take off the windshield.

[SmithSwede]

I was recently futzing around with a clutch master cylinder on an old Ducati.  Trying to be cheap, I kept bleeding it.  Then I looked for a MC rebuild kit.   Finally I found a brand new OEM master cylinder with lever for $89.  Discovered that brand new Galfer lines were also surprisingly cheap. 

Replaced the old stuff and everything worked perfectly. 

You might just want to bite the bullet and replace the suspect components. 

[Ryan]

So it has never worked correctly. Maybe it was manufactired without that hole being drilled, or in the wrong place. Since the locking brake is what kept the original owner from ever riding it, just replace it and be done. If it turns out that the new one looks just like it, then clean up the old one, make it functional and get some of your money back on Ebay.

[SIR REAL ED]

I can't say for sure that it never worked properly, or if someone rebuilt the caliper improperly.

I think I found the problem.  It looks to me like both seals were installed to pressurize the caliper.  It seems to me, that the seal closest to the return spring is backwards.  It’s function should be to pull brake fluid back into the reservoir.  Also, for some reason the thin brass washer was installed against the piston where it serves no purpose, not against the spring where it would provide some axial support for the return seal.  See below.  Upper image is what I found.  Lower image is how I think it should have been assembled.

Surprisingly, the front brake line is braided stainless steel (always a good mod) while the rear brake line is the original rubber line.  Maybe someone decided to do a front brake upgrade and only got it half right.






upload a picture

[BMCMOTO]

Both seals should face the mc outlet. That is the larger diameter should face the direction of applied pressure. First picture is correct.

Brian

[SIR REAL ED]

Both seals should face the mc outlet. That is the larger diameter should face the direction of applied pressure. First picture is correct.

Brian

I think you are right.  I was assuming since I could not find the reverse bleed port, that it did not exist.  I was looking for a much larger hole than the one that should exist.  Based on one video I saw, the bleed port may only be about 0.0135" in diameter.  Time to take a couple measurements and see if I can find the bleed port from the piston bore.  I think I know where it is, but it is not viable.  Or just ending the tinkering fun and buy another master cylinder.

[wirespokes]

My recollection is the large thin washer goes between the seal and the holes in the piston.

Yes, the bleed-back port is very tiny - about the diameter of a pin. I'll bet it's clogged.

[SIR REAL ED]

My recollection is the large thin washer goes between the seal and the holes in the piston.

Yes, the bleed-back port is very tiny - about the diameter of a pin. I'll bet it's clogged.

That's where I started to delude myself.  All the other master cylinders I have messed with, the bleed hole was about 1/4 the diameter of the larger port.  The larger port is about 0.120" in diameter.  Even though the master cylinder was pretty clean, there was so visible port (at least of the size I was looking for).  I was not looking for a pilot jet sized hole.

I know where the bleed port has to be, so I should be able to find it now.

As I often say, the human mind has an infinite capacity for self-delusion.  But hey, If you can't delude yourself, who can you delude?

[Huzo]

Can you blow low pressure air through the m/c with the piston in the retracted position ?
If you can, it’s not blocked.

[SIR REAL ED]

Can you blow low pressure air through the m/c with the piston in the retracted position ?
If you can, it’s not blocked.

No movement of air or brake fluid in the reverse direction was possible at first.  It was definitely blocked.  But it is not now.  I'm not sure what the original bleed back port diameter was, but it is 0.032" now.  Hooray for tiny drills and little old fashion "egg beater" type hand drills.

Oddly enough, I could not see it from the reservoir side, even thought I knew exactly where it should be.  But I was able to see it from the bore side, and feel it inside the bore with a pick.

So far the bike is doing exactly what I bought it for, to be a good platform for tinkering.  Great recreation!

[Chuck in Indiana]

So far the bike is doing exactly what I bought it for, to be a good platform for tinkering.  Great recreation!

And.... it's a good feeling to "fix" a mysterious problem. 

[wirespokes]

And.... it's a good feeling to "fix" a mysterious problem. 

Especially when several others gave up - in fact selling the bike as-is and not even letting you know there was a problem. Pretty funky finding out about a dangerous thing like that on your own.

I pretty much expect issues with used stuff I get, whether a yard sale find or a used bike. The challenge of making it right is fun - puzzles without cut-and-dried solutions that force you to think outside the box.

[SIR REAL ED]

And.... it's a good feeling to "fix" a mysterious problem. 

Especially when several others gave up - in fact selling the bike as-is and not even letting you know there was a problem. Pretty funky finding out about a dangerous thing like that on your own.

I pretty much expect issues with used stuff I get, whether a yard sale find or a used bike. The challenge of making it right is fun - puzzles without cut-and-dried solutions that force you to think outside the box.

I never mentioned it, but the previous owner did tell me that the front brake locked up which is why his friend crashed the bike.  At least that is why the friend said he crashed the bike.

I am of the same mind set, whenever I buy a used bike, everything gets checked, tightened, adjusted, and fluids replaced.  Doesn't matter if I plan to keep it a long time or just tinker with it.  It is a mechanical device, so there is no excuse for not making it right.

It would have been smarter of me to look inside the bore for the return port, rather than just assuming because I did not see one from the reservoir side, that the return port did not exist.

Good part about admitting your mistakes is that you learn from them.