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i have built mrs 1111 a bike with old parts that i and you (stw) had lying around.
all is good bar the back brake, every time i pull the brake its fine then slowly fades and will pull back to the bars.
at 1st i though it was air so i bled it. same again so i bled it again, but still the same. any ideas??
many thanks shaun
make sure you let new fluid settle for 48hrs before you bleed the system (to get rid of tiny bubble in the new fluid).
tap the lines to ensure bubble rise up them & get out of the system.
check the fluid is at the right level in the reservoir (do avids have a reservoir?) & that you are not pumping air in with the lever action.
watch the pistons to make sure they are moving freely & that no fluid is bleeding out of the seals or caliper join
Is the fluid straw coloured like Stella or darker like bitter? Dot 4/5.1 Fluid turns dark as it absorbs water - this might indicate you have a leak but it's sucking in water/air instead of expelling dot fluid.
HTH
If there's no fluid leaking out of it then you haven't bled them right.
Could be there's some air trapped by the piston. Try gently pulling the lever back to the bar when you're pulling a vacuum on the resevoir. Release it when you let the vacuum go and gently push some extra fluid into the resevoir.
tryed all of that
no leaking
at 1st it feels ace then 3-4 pulls on it then its back to the bars!!
Oh should have added. Make sure that you de-gas the DOT fluid properly before using it.
You'd be amazed how much 'gas' gets absorbed by DOT fluid.
ETA: Are you pushing the calliper pistons all the way in before you bleed? Sounds like you've still got air in the system. Or the seals on the lever piston have gone - but then fluid would be leaking out.
Are you squeezing extra fluid back into the resevoir before you finish? you should feel the diaphragm pop into the place.
how do you de-gas dot??
is that like finding a left handed screwdriver
The Avid bleed procedure uses syringes. You lock off the syringe then pull down on the plunger to form a vacuum in the syringe. You do it 20 / 30 times. Then open the syringe and push out the gas. Youtube Sramtech and you'll see them do it as part of the Juicy bleeding film.
This is what I always follow:
[url=
Tube Avid Brake Bleed How-to[/url]
Different model of brake but very similar I beleive?
The Syringes are the critical bit of kit. Massively over priced but don't know how you'd do it without them.
Any help?
will have another go!! will let you know what happens
You can do a DIY bleed kit for alot lot less money but needs a bit of your time to organise. Personally, I'd rather spend the money than spend three hours organising it.
Sounds to me like the primary seal in the master cylinder is leaking,won't show up as an external leak as the fluid will just bleed past the seal into the reservoir giving that sinking lever feeling you've got.
Wot coatsy says - they symptoms are not those of air in the system. Master cyclinder seal is likely if the lever slowly goes back to the bar
+1 for coatsey, happened to mine.
Sounds to me like the primary seal in the master cylinder is leaking,won't show up as an external leak as the fluid will just bleed past the seal into the reservoir giving that sinking lever feeling you've got.
Would, as a hypothetical question, squeezing / pulling the syringes a bit much when bleeding cause this?
Just wondering...
Just a quick thread hijack:
"Is the fluid straw coloured like Stella or darker like bitter?"
What if it happened to come out a dark gray colour?
I can't find it leaking anywhere, though I've just had to bleed it again after about 4 months of working fine before pulling closer and closer to the bar over the course of about 3 rides
coatesy - Member
Sounds to me like the primary seal in the master cylinder is leaking,won't show up as an external leak as the fluid will just bleed past the seal into the reservoir giving that sinking lever feeling you've got.
Are you talking about the diaphragm in the resevoir? If you are then it would leak out of the air hole in the resevoir cap.
Or are you refering to the first seal on the piston leaking and the second seal then doing all the work as it passes the resevoir? Would an indicator be the lever not feel rough as it's pulled?
James, The fluid has got dirty through use. Flush it through with fresh fluid. I've flush black fluid out of my brakes before.
Another +1 here for master cylinder seals. I had the same problem, sounds like exactly the same symptoms.
Air in the system or an external leak would feel totally different.
mk1fan -> not the diaphragm, no, the seals on the actual piston. It only takes the tiniest imperfection in the seal for it to happen. You won't neccesarily feel roughness, but sustained pressure on the lever will evetually start letting fluid back past the seals in the wrong direction which lets the lever pull to the bar. A quick sharp few seconds of braking doesn't usually cause a problem though.
i did a guide to how to bleed formulas with the avid kit somewhere on here, it sounds like you've not bled it proper 🙂
Theres a leak in there somewhere, you've just not found it yet.