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Discussion Starter #1
No, it didn't fall off the bike. Using the foot pedal does not cause any perceptible slowing at all. The front brake works as usual. Needless to say, the pace was a lot slower coming home.

The linked brakes should cause two of the front pistons to activate when applying the rear brake, but I couldn't detect it.

The rear reservoir is empty. The front reservoir is full. No detectable leaks anywhere.

Yes, I could fill the rear reservoir and bleed the brakes, but I would feel a lot better about it if I could explain what happened to the brake fluid that was in the reservoir. The last time I checked it was the last oil/filter change - about 2000 miles ago. NOTE: I did not change the crush washer during that oil change.

Suggestions welcomed and appreciated.
 

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As the brake pads wear, the caliper piston has to move farther, hence the level in the master cylinder will go down.
 

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As the brake pads wear, the caliper piston has to move farther, hence the level in the master cylinder will go down.
I would think that the brakes reservoirs are sized so that there will be sufficient fluid in them to prevent the introduction of air into the lines, even with the all the pads worn down to their limits. Maybe not.

If the front pads were worn down the fluid level in that reservoir would also be low. That's why when you install new pads and push the pistons back into the caliper bores, you have to be careful that you don't overflow the reservoir.

Or perhaps a bleeder valve was not quite tight enough and allowed a very small amount of brake fluid to escape on each use of the brakes. Tho it would have to be a VERY small leak not to allow brake pressure. Or there could be a problem with one (or more) of the caliper pistons allowing fluid to escape around the piston bores.

You're absolutely correct about wanting to know the real reason for no brake fluid in your rear reservoir. Merely refilling the master cylinder and bleeding is not going to solve the REAL problem: A brake system leak.

Thank goodness you caught it before it caused a major accident.
 

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Discussion Starter #6
I put the crush washer comment in there to short-circuit that "cause". It worked, sort of. :)

When I say no visible leaks, I looked very closely. The underside is dusty, as you would expect on a bike that's ridden more than washed. Any leak would be very obvious, especially one that completely emptied the rear reservoir.

The pads are good front and rear and do not need replacement, they're not really even close to needing replacement.

This bike does not have ABS, if that is helpful.

Any other suggestions? Thanks in advance.
 

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You can wear the pads to the metal and still have plenty of fluid in the tank. Rear brake line also goes up to a proportional valve up front and are linked to the antidive also. You might want to check up around the front and see if anything leaks. I have never seen brake fluid evaporate so it had to go somewhere.
 

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Brake fluid does not evaporate. You will need to either locate that leak or assume that it was not filled correctly and as the pad wore down, emptied the resevior. Start with doing a good visual inspection looking for a leak and don't forget to inspect the pads.
 

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Could it be the PCV behind the left radiator? Could be leaking and you not notice.

Techdude2000 beat me to it.
 

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Check out the PCV, proportional control valve. Happened to me and that was the culprit.
 

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You were concerned about the reservoirs accommodating all pad wear.... Well if you look at that reservoirs and compare them to the 1200's and the 1500's you'll notice they are the same size... Now with the 1800 brake calipers being larger don't you see where it is possible that you simply need new pads (or to add fluid).??? Have a look at all the pads (fronts too) and I think you'll find they are getting thin... Once you compress the caliper pistons back in (all of them) you'll likely find the reservoir once again has a good level of fluid in it... If not then you surely need to find the leak...
Just a note on the caliper thing, do check the reservoir (both ones) after each caliper is done as you'll likely over flow the reservoir by the time you get to the third one...
 

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Discussion Starter #14
You were concerned about the reservoirs accommodating all pad wear.... Well if you look at that reservoirs and compare them to the 1200's and the 1500's you'll notice they are the same size... Now with the 1800 brake calipers being larger don't you see where it is possible that you simply need new pads (or to add fluid).???
I'm going to pull the top shelter and get a look at the PCV. I don't think it's the culprit, but I need to be sure it's not leaking fluid.

The front brakes are unaffected, and work fine.

But in thinking about the fluid level and the pads wearing, the rear reservoir level will be affected by the rear pads AND the front pads. I suspect that the issue is an improperly-filled rear reservoir and pad wear on all three calipers.

If it cools off tomorrow, I'll have a look under the top shelter and see what's going on.

I'll post back with what I find.
 

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If it is a significant leak... just fill, purge system, run the bike on the center stand and mash down the the brake pedal a bunch of times.

Shut er down, check your fluid level. Even if you can't see fluid dripping (which I expect you would) you would notice the fluid level being lower.

Faster than doing top shelter removal?
 

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Discussion Starter #16
If it is a significant leak... just fill, purge system, run the bike on the center stand and mash down the the brake pedal a bunch of times.

Shut er down, check your fluid level. Even if you can't see fluid dripping (which I expect you would) you would notice the fluid level being lower.

Faster than doing top shelter removal?
I think at this point I'm looking under the top shelter for peace of mind and confirmation, not because I expect to find the problem. Unfortunately, I've had a lot of practice removing the top shelter - takes about 15 minutes or so.

When it cools off I'll see what I can learn under there.
 

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Dale,

I hope you don't have a leak and it is caused by the pad wear. For some reason though, I find that hard to believe. If your rear master was full when you checked it last 2000 miles ago, I don't think pad wear would drain it that fast. Have you ever topped off either of the reservoirs?

One thing does enter my mind though. Have you ever changed just one set of pads. Maybe on the left front? Usually the left front will wear down the quickest, leaving plenty of pad on the other two. If that were the case, you would be getting the most from all three sets of pads. But that still doesn't change the fact that the master was full 2000 miles ago.

Let us know what you find.
 

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I think at this point I'm looking under the top shelter for peace of mind and confirmation, not because I expect to find the problem. Unfortunately, I've had a lot of practice removing the top shelter - takes about 15 minutes or so.

When it cools off I'll see what I can learn under there.
Dale.The PVC valve is behind the left radiator.(it's tucked under/behind there).
Try removing the left pocket first and see if you can follow the brake lines to the PVC valve to see if you can detect and wetness in the area?.

I don't think pulling the shelter will give you that much of a better view.A small mirror might be needed to look around in that tight space.

If you have a service manual,look on page15-39.
 

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Discussion Starter #19
UPDATE: I found no leaks around the PCV so I buttoned the top back up.

I used my motion pro bleeder on both bleed points on the rear caliper, got some air (which is a bit of a surprise) but even after there was no air bubbles in the fluid the pedal still goes to the bottom without applying the brake.

I"m beginning to think the rear master cylinder is bad, that would explain the lack of rear brake force without there being any fluid leaks. I have ordered pads all around and I'll install them, flush the old fluid, and bleed the system per the sequence in the manual.

I have a suspicion that the problem will still be there, and the next option is to rebuild the rear master cylinder. Or do you have to replace it?

Suggestions are welcome.
 

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UPDATE: I found no leaks around the PCV so I buttoned the top back up.

I used my motion pro bleeder on both bleed points on the rear caliper, got some air (which is a bit of a surprise) but even after there was no air bubbles in the fluid the pedal still goes to the bottom without applying the brake.

I"m beginning to think the rear master cylinder is bad, that would explain the lack of rear brake force without there being any fluid leaks. I have ordered pads all around and I'll install them, flush the old fluid, and bleed the system per the sequence in the manual.

I have a suspicion that the problem will still be there, and the next option is to rebuild the rear master cylinder. Or do you have to replace it?

Suggestions are welcome.
Dale.The early bikes had defective rear master cyl's.But it can also happen to the later model bikes a well.

You won't know if yours is rebuildable unless you take it apart and inspect it's bore for pits.

FYI.
I just rebuilt a 30 yr/old master that was good to go.So your 06 should only need a rebuild kit.
 
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