I appear...

.... to have some semblance of brakes now. Spent yesterday fettling and
fiddling, examining all the joints, calipers, pistons, hoses et. Culminated
in the simple act of tying the master cylinder in elevated position, cable
tying lever, rocking bike from side to side and tapping all the joints and
hoses.

This morning snipped off the cable tie to find that I had a nice firm lever.
It's settled back in a bit again, so I'm repeating the process tonight.

<relieved>

 

Tying the lever back overnight does usually help. Bleed, tie back, bleed
again. Short of finding a shop with a pukka pressure bleed system who is
prepared to flush your system right through ya just have to keep at it.

Does it have goodridge lines? and even if it's the older type some new
microbores might help if nothing else improves feel.
Did Loz replace the seals at the master cylinder as well as the
callipers.

I had a K100LT which I was *sure* allowed air back into the system by
some devious unexplained route. Never did get it completely sorted.

'Hog

 

Yes, he did. He replaced the dust seals, the fluid seals and the
master cylinder seals. The system was bled vertically, horizontally
and every other fucking way. It was fine. The amount of fluid it has
had through it doesn't bear thinking about.

 

Pip says...

Ditto. Dunno what, if anything, I did wrong. When I left here, Pip will
attest to this, they were absolutely excellent and the lever had about
1/4 inch of play at most.

 

And it's now back to that after repeating the process last night.
Don't think for a moment you did owt wrong, suspect it's just needed a
bit more after a few runs. It had slowly degraded to the stage of the
lever only firming up approximately 1" from the bar. There's no fluid
escaping anywhere so my guess is microbubbles.... or that you've
surreptitiously taken out a massive insurance policy on my life.....
;-)

 

Yes I knew this last bit. It's really weird how something as simple as a
hydraulic system can vary this way. As Bee says, microbubbles, but how
do the fuckers get back in there. It has to be past a seal somewhere,
moisture boiling up or a new quantum migration model. Perhaps Ivan could
turn that new toy of his on this problem?

'Hog

 

Ahhh, but I don't think the microbubbles get back in as such, I think
what happens is that they're already there but originally so tiny that
they're not noticeable. However, over a period of usage, they migrate
and gang together forming bigger bubbles, which become very noticeable.

Of course, I could be talking rubbish!

 

Hmm do different size air bubbles behave differently? good question.
Over to Ivan again really.
It could be gamma ray strikes through the liquid causing ionisation and
possible gas bubble generation.

'Hog

 

Well, the bigger the bubble, the more of a void it will
present,increasing apparent compressibility. In an incompressible
fluid, if you have lots of teeny-tiny, independent bubbles , the
compressibility of each tiny bubble has much less of a total effect,
than if they were clumped together in one big bubble.

Badly explained, I know, but I know what I mean.

 

Except all fluids can be compressed of course ;o)

So what do we think tying back the lever overnight acomplishes?

'Hog

 

We don't fucking ask, of course. It just fucking works, OK?

 

Inquiring minds need to know!
I suspect it compresses the fluid slightly creating an environment that
the nanobugs don't like and they migrate temporarily into the fork oil.

'Hog

 

'Hog scribbled:

I recall on a Fiesta they had problems with the brake pedal always
having loads of travel, after "fettling" it would be OK for a day or
so, and then back to crapness.

Turned out to be one of the front brake discs was not bolted on true
(bit of grit underneath it), so as it rotated round it wobbled the pads
in and out when the brakes were applied, causing the fluid to be pushed
back into the master reservoir, causing lots of pedal travel (or having
to pump the brakes at worst) when they were next applied.

Took the garage bloody ages to find that one

Dunno how likely that could happen to a bikes braking system with
floating discs (or even semi floating). I suppose if the disc was
warped it could have the same effect?

 

'Hog scribbled:

Forces the bubbles to be absorbed back into the fluid, like carbonated
liquid, innit?

Hence shaking your bike vigorously will reduce performance of the
brakes a lot quicker than riding it sedately.

 

Beelzebub@Work says...

It might have been down to the brake fluid I used. It was always clean
new fluid stright from the bottle, but it was just average cheapo from
the local car parts emporium. As a last resort I'd go for summat like
Goodridge's own stuff to see if that makes a difference.

 

Champ says...

Like I said, it's a last resort. I've done absolutely everything, new
seals all round, new master cylinder seals, new Carbone Lorraine pads
the fucking lot. The only things I haven't changed are the brake lines
cos they are braided ones and I didn't use expensive fluid.

 

Champ says...

You reckon?

What makes you say that? They will bleed up fine then go back to where
they were after a couple of days by the sound of it. If it was down to
discs then they'd go back to previous form as soon as you start riding
and using the brakes. There's no judder from the brakes either, so that
rules out warping.

 

I had the same problem on the Tig (which has braided lines). Stapping
the lever to the bars overnight would see it fine for a full day's
riding, but no more than that. Castrol SRF cured it, but don't ask me
why.

 

Krusty says...

May be worth a try with a decent brand off fluid, rather than the Comma
stuff that's in it at the moment. Thanks.

 

I've never had a problem with that in the past but I have had bleed
nipples and banjo bolt washers that were worn enough to allow air to be
drawn in via the threads, but not for fluid to leak out.