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Brake Bleed!


JimMoore

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Hi Guys,

 

I just got done replacing the brake fluid on my '98. I'll share my lessons learned. First, you need a MityVac or similar for this job. I've found that the trick to the MityVac is as follows. Squeeze it once or twice to set up a slight vacuum, then gently pump the lever (or pedal) to force out the fluid. The slight vacuum holds the old fluid in the hose. You may need to go back and forth several times between the MityVac and the lever. 

 

For the fronts, the "outside" nipples (at the top of the caliper) are bled like every other set of brakes. Fluid is pulled from the front reservoir. Simple. The "center" nipples are bled from the rear system. You need to pump the rear brake pedal, AND you need to refill the rear reservoir as you bleed. It doesn't feel quite normal , probably because the fluid is going God knows where on its journey from the rear reservoir to the nipple. Btw, you don't need to take the rear fairing off to refill that reservoir. Remove the screws holding the right side in place. That will give you plenty of room to work.

 

Next, the PCV valve. The book says to move the tank forward out of the way. My tank was completely off so I didn't need to do that but it's probably necessary. The PCV valve bleeds from the rear reservoir and uses the rear pedal.

 

I had the rear wheel off for the rear brakes. I'm not sure if it's required, but it made it easier. The outside nipple is the "standard" line. It bleeds from the rear, as you would expect. The center nipple is impossible to reach. I pulled the caliper (Two 12 mm bolts. One easy to reach, The other hidden up by the hoses) and jammed a screwdriver between the pads to hold them open while I bled. It bleeds from the rear, using the rear reservoir and the rear pedal.

 

Anyway, nothing too hard, just a little more complicated than a typical bleed. My main surprise was that there was a LOT of air in the linked part of the system, the center nipples and the PCV valve.

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I do mine every (off) season in the '98 and seem to find a slightly different/better way each time. I'm surprised that you had brakes at all with any air in there. You might want to hunt down how that happened. (Are you sure it wasn't getting in around the nipples while you were bleeding?)

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1 hour ago, bmart said:

I do mine every (off) season in the '98 and seem to find a slightly different/better way each time. I'm surprised that you had brakes at all with any air in there. You might want to hunt down how that happened. (Are you sure it wasn't getting in around the nipples while you were bleeding?)

Yeah, I'm sure it was in the lines. I've done it wrong enough times to know what it looks like. All the air was in the linked portions of the system. Maybe my linked brakes have never worked and I just didn't know.

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Sounds like you had better luck with the MityVac than I have ever had. I gave up on using it, as I could never get stop air getting

pas the threads on the bleed screws with using some kind of sealant. And I didn't like the idea of some contaminant getting into

the system. In truth, it wasn't that bad of a deal to bleed them, but full disclosure, I didn't do the PCV valve. And I did it when I

replaced all the rubber lines with stainless.

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The hardest part about bleeding an LBS is the SMC - a vacuum bleeder will never pull fluid through it.  The only way of doing that is forcing it through using the rear brake pedal.  The trouble with not changing that portion of it out comes with the small filter screen and a tiny port in the SMC (there's a defunct thread on here about this) that will plug with decayed fluid.  When the SMC applies pressure to the rear caliper the pressure will not release and a dragging rear brake results.  Tho it seems inconsequential, changing out that portion of the system is needed for its long term correct functioning.  The pinned topic on bleeding 6th gen systems covers it - different but similar on 5th gens. 

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6 minutes ago, Cogswell said:

The hardest part about bleeding an LBS is the SMC - a vacuum bleeder will never pull fluid through it.  The only way of doing that is forcing it through using the rear brake pedal.  The trouble with not changing that portion of it out comes with the small filter screen and a tiny port in the SMC (there's a defunct thread on here about this) that will plug with decayed fluid.  When the SMC applies pressure to the rear caliper the pressure will not release and a dragging rear brake results.  Tho it seems inconsequential, changing out that portion of the system is needed for its long term correct functioning.  The pinned topic on bleeding 6th gen systems covers it - different but similar on 5th gens. 

How do you bleed the SMC? I may have missed that.

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Check out this thread in the maintenance guides forum.  Purists will say that 6th ad 5th gen LBS systems are different - and yes, they are.  However, the concepts are similar - at the end there is a detailed description of bleeding the SMC.   I've done it on both my 5th and 6th gens (ABS) - it only varies slightly but the concept is the same - force the fluid through he SMC and extract it through the PCV.  Looking at schematics of the 5th gen system may help visualize it. 

 

https://www.vfrdiscussion.com/index.php?/forums/topic/70842-bleedingfluid-replacement-on-linked-brakes-wabs-on-6th-generation-the-ultimate-guide/

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This is a good tutorial on the SMC and what can happen when fluid in it goes bad.  The diagram was referenced as being posted by a member I cannot find - but as I recall in that post he had bought a badly neglected 5th gen with this issue and finally sorted it all out, finding that the green filter was clogged.  Unfortunately I cannot find that member or post - possibly it was deleted.

 

 

https://www.vfrdiscussion.com/index.php?/forums/topic/93973-rear-brake-locking-up-rear-center-piston-fixed/

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Got a vac thingy to do mine and struggled with it. I think ended up using it a bit to get the first air out perhaps like you say, but found it no use to get all air out or to replace the fluid as I think air kept getting back in.

 

I used one of those one way valves in a line (like a speed bleeder, but not permanently fitted to the bike). Found it brilliant for pushing out old fluid with remaining air from the different bleed nipples. It means you just need to keep en eye on topping up reservoir and pumping lever rather than what’s happening at the nipple. But I recall I did need to clear worst of the air at caliper before I could get any fluid out of the nipple to prime the one way valve.
 

Empty and clean reservoir before starting of course, but the combination of those things made the job not too bad at all (whereas one or the other I was getting nowhere).

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