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Old 04-Jul-2006, 01:01
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mara mara is offline
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Question Bleeding clutch

You'll soon work out I'm not an experience spanner man, so please bear with me.

After a great, long ride out on Sunday, this morning on the way to work the clutch on my ST4 packed up - the fluid resevoir was empty. I have read on the net that leaking slave cylinders are quite common, so I suspect this.

In order to get me moving again, I bought fluid and a bleeding kit. I read the instructions and expected a very straight forward process.

I slackened the bleed nipple on the top of the slave cylinder and attached a bleed pipe. I filled to the max mark with fluid and pumped the clutch lever. I expected air and then a flood of pumping clutch fluid. However after much pumping the lever, I got an unconvincing trickle of brake fluid.

I'm not sure what I'm doing wrong, I tried it with and without the top on the fluid resevoir, with the black rubber insert (what's the purpose of this insert, I assume it's removed to fill the fluid).

After several attempts the clutch is still not working as I think the line is full of air.

Any suggestions welcome, do I have to fix the slave cylinder seal before the line will bleed properly? Could it be a problem with the master cylinder?

I would welcome some guidance as its now July and the bike is stuck in the garage.

Many thanks.
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Old 04-Jul-2006, 01:20
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Glyn Glyn is offline
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Mood: st2......we can rebuild
leave the bleed nipple tight
pump the lever 7-8 times and hold in
slacken nipple and retighten, just leave it open for a second or so
with nipple closed repump lever
and repeat
as yhe fluid starts to replace the air the lever will start to firm up again
if the system is full of air this might take a while
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Old 04-Jul-2006, 10:22
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DSC Member Jools Jools is offline
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Yes, you'll have to fix the slave cynder seal before bleeding the clutch through again otherwise it may bleed through but you'll have the same problem very quickly. You'll probably find the easiest way is to replace the slave cylinder completely because the cylinder bore is often scored. If your current slave is sort of flat topped with a smaller slightly raised boss in the centre of it you have the old type slave. Replacing this with a newer design slave (one that's got a big star shaped 'torx' head in the centre) will be much more reliable - the design is improved so that road dirt and crud from the chain don't get into the slave so easy.

Once you've sorted that out you can bleed the clutch. I've found that the problem with clutches is that once the system starts to get primed the slave cylinder starts to move (this is of course what you want but it makes bleeding more difficult). As Glyn says, the system will also be full of air.

You do want to take the rubber diaphragm out of the reservoir. This is here because you do need atmospheric pressure to act on the fluid in the reservoir, otherwise as the fluid level dropped as the clutch self adjusts there would be a vacuum in the reservoir stopping the fluid getting through to the master cylinder. However, brake and clutch fluid is very hygroscopic and absorbs moisture from the air. You don't want any water in the fluid because under the heat of the clutch this will boil off and effectively give you the same problem as if the clutch was full of air. So take it out to bleed the clutch and don't forget to put it back in.

I've found that with a completely empty system the master cylinder doesn't want to pump fluid until you've primed it first. To do this, get a whole heap of absorbent material under the banjo at the master cylinder then just crack the banjo bolt open until fluid can just seep out. Squeeze the lever SLOWLY so that fluid just seeps out between the cracks and doesn't spray everywhere. When the lever is back to the bar, tighten the banjo and release the clutch lever. Repeat this process three or four times and the master cylinder should now be free of air.

Next, repeat Glyn's instructions and you'll drive any air out of the line. I've sometimes found that priming the slave cylinder helps as well. I do this by taking the slave off the bike, but leaving all the hydraulics connected. I squeeze the clutch lever so that the slave piston extends, then l let go of the clutch lever, open the bleed nipple and squeeze the slave piston back in with my thumb, again you only usually need do this two or three times.

Hope that helps and good luck


Oh yes, and don't forget that hydraulic fluid is nasty stuff that'll eat paintwork and doesn't do your skin much good either, so wear latex gloves, have plenty of rags around wherever it might splash and have plenty of bike cleaner and water standing by to sluice any splashes off ASAP. But don't do what I did once - faced with a spurt of fluid that hit my petrol tank I grabbed the hose and rinsed it for ages. That was fine until I realised that I'd grabbed the hose in panic before I had put the top back on the fluid reservoir, I'd then got water into the reservoir, contaminated the fluid and had to start all over again.


The Patent Jools Mood Meter -Today I am:


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Last edited by Jools : 04-Jul-2006 at 10:29.
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Old 20-Jul-2006, 20:03
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Thanks for the help. I have just got back from holiday and the seals have just arrived through the post. I'll be fitting them tomorrow, if I get stuck, I'll be back.
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Old 20-Jul-2006, 23:14
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OK, I've just had a go. Fitted a new seal, primed the master cylinder OK and followed Glyn's instructions to prime and repeated many times. Fluid seeps out (no air), but very slowly and there's definitely no progress.

When I took the slave cylinder off, the slave piston stayed in the bike, it didn't come out with the hydraulics. I tried to remove it gently and it wasn't moving, I didn't want to force it. This doesn't seem right and means I couldn't prime the slave cylinder.

Any more ideas?

Thanks.
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Old 21-Jul-2006, 17:13
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The slave will be full of air too. With it off the bike, open the nipple, push the piston in as far as it will go, tighten the nipple and release the piston. Do this 2 or 3 times and hopefully you will get there. Once the piston starts to move I found it useful to release the nipple with the lever held in. The clutch pushes the piston back and purges the air out of it. Tighten the nipple before releasing the lever or it will draw air back into the line.
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Old 25-Jul-2006, 06:54
ColinF ColinF is offline
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I kept this letter to the ed of Classic Bike when I found it some time ago because I've had the same probs you've had and this guy seemed to have it sussed
".....my experience after 30 years in the old bike business?Bleeding brakes the simple quick and cheap way.
Required: cheap plastic or glass syringe,preferably greater than 100cc.The bigger it is the fewer times you have to empty it.

Although it was not mentioned in the article,the design of the bleed nipple with the bleed holes in the side above the bottom taper means that a certain amount of air will be sucked down the bleed nipple threads when using any vacuum bleed sytem.I always remove the nipple first and wind a couple of turns of plumbers tape around the threads, just enough to seal.Be careful not to cover the small bleed holes above the bottom taper.

Screw the nipple into the caliper until it bottoms and back off around one turn.

Use syringe to suck fluid through system, maintaining fluid level in master reservoir.
Continue until fluid is no longer aerated.Lightly tapping lines, master, calipers etc as you draw fluid throught the system ensures a quick, highly effective bleed time and time again.

Tighten bleed nipple.Thread seal tape can be left in situ.It is actually best to leave it there as it seals the threads, minimizing water entry and the dreaded expensive 'broken nipple' syndrome!

To rebleed the system merely loosen nipple again and use the syringe.

A couple of pounds for a syringe is a much more economical proposition than a vacuum pump.

Occasionally customers have brought in bikes with brake systems that have defied their every efforts to bleed by the usual pump and release method. I love the looks on their faces when they see the job done to perfection in five minutes at the most.I refuse to charge them and give them a syringe to take home."

I haven't needed to try it yet but it reads well eh?
Goodluck
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Old 30-Jul-2006, 00:11
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Thanks for all the suggestions, I sorted it last weekend, the trick was to prime the slave cylinder. Doing this moved air much quicker and soon made the lever pumping method more productive. I haven't tried the syringe method, but it sounds good.

Tips for novices like me:
1) Prime the master and slave cyliner following instructions above (I found the easiest way to compress the slave piston, was to have it loose on the bike and pushed the whole slave unit onto the bike to compress the piston).

2) It really helped to have the drain tube I attached to the nipple and bottle to catch the fluid above the bleed nipple, this means the air floats up the tube, reducing the risk of drawing back into the system.

3) Keep an eye on the master cylinder and make sure the reservoir doesn't empty, otherwise you'll draw air back into the system - this is very easily done.

3) Be patient, it takes much longer than you'll expect (maybe the syringe method is the way ahead).

I'm back on the road.
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