Technical bleeding clutch - twinair

[farmer.palmer]

So the clutch in my 2012 twinair got a bit sticky, and reading around the subject it appears as though both master and slave cylinders give problems so I thought I'd replace them both at the same time.
Ordered the bit both turned up as the correct parts, actually fitting them both took less than 1 hour. I then tried to bleed the system - nightmare.

I have a pressure bleeder which I run at very low pressure, around 5psi. When I opened the bleed valve on the slave cylinder the liquid seemed to pour out from around the base of the plastic housing rather than out of the bleed nipple and into my carefully arranged pipe. No problem, I'll vacuum the fluid thrrough, same issue I can hear it pulling in air around the base of the bleed nipple housing. I then condemed the new slave cylinder as faulty and refitted the old one, which did exactly the same thing.
Anybody else experienced this? It seems as though the little hex screw, when opened fractionally allows air or liquid to flow out around the where the plastic housing meets the body of the cylinder.

 

[Mike1alike]

That behaviour is quite common on these slave cylinders. The bleed screw doesn’t always seal perfectly when opened, so fluid can come out around the base of the plastic housing instead of only through the nipple. With a pressure or vacuum bleeder that often pulls air in around the threads, which makes bleeding almost impossible.
Usually the easiest way is the traditional two-person method (pedal down → open nipple → close → pedal up). The pedal pressure tends to push the fluid out faster than air can leak in.
Some people also wrap a little PTFE tape around the bleed screw threads to stop air being drawn in when using a vacuum bleeder.

 

[jrkitching]

I find that vacuum bleeding is the least worst option, but even then, some fluid always ends up where it's not wanted. It's much less messy than pressure bleeding, and not quite as messy as pumping the pedal. It'll still leak a bit of fluid, but that's not nearly as bad as spraying it everywhere under pressure.

It'll pull a lot of air through the thread gaps, but that won't affect the quality of the bleed. You'll need to discard whatever you pull through, but you should be doing that anyway. Exactly the same thing happens when you vacuum bleed the brakes.

The reservoir is quite small, so you need to keep topping up regularly, whatever method you use.

As Mike says, a bit of PTFE thread around the threads can help, but you'll still get quite a bit of air in the receiving container. It helps to have a decent bleeding tool; you need to maintain a good vacuum for the best results.

The main disadvantage of vacuum bleeding is that you won't be able to use the absence of bubbles as a check that bleeding is complete.

 

[farmer.palmer]

Thanks both your your replies, not having a able assistant to hand that day, I ended up wrapping PTFE around the threads of the nipple and around the base of the plastic 'turret' the bleed nipple sits in, and vaccum bled it very patiently (not something I am known for!). Got there in the end, but you're right liquid everywhere, when I can normally bleed most things with just a lost drip or two of fluid.
What a ***** design.

Thanks for your help.

 

[system11]

Exactly what I had to do a few weeks ago. I put a great big rag there to catch all the fluid since any controlled attempt was futile. Pedal, nipple, pedal, nipple, repeat.