It’s also notable that there aren’t any exterior seals for the lever piston to stop mud, dust and debris entering the piston – granted the end of the piston is in a fairly sheltered position, but its exposed and I suspect this is the main factor for dirty oil predominantly located in the lever.
Yes, this makes sense. It would be much easier for fine dust to work its way past the master cylinder piston seals than to get past the wheel cylinder seals.
A couple of pics - 4 year old zee lever piston. Youll need to zoom in to see the wear marks on the body.
The exposed end in an m785 lever. AFAIK all shimano brakes of this type are similarly 'unsealed'.
AFAIK all shimano brakes of this type are similarly ‘unsealed’.
Technically, there is a seal there, otherwise the fluid would just pour out. It's just that the seal is designed to keep fluid in, not keep debris out. If you blast it with a pressure washer, you could easily blast dirt into the seal. Even if it wasn't enough to get it past the seal, it could work its way past with repeated movement of the piston.
Yes. Thanks for clarifying my point.
Does anyone know how thick the pistons are? What I mean is, how far can you allow them to extend before they pop out completely? If cleaning them and then lubricating with silicon is advised, what is the cut off point for pushing them out? Does anyone have a pic of a 4 pot piston that has been removed from the caliper? I'd be very interested to see one...
Ime dot fluid gets filthy as the rubber diaphrams degrade. And before anyone says "that's not true" it certainly is.
Guides/codes suffer with the lever bladder degenerating whilst the piston seals are ok.
I'd guess half the thickness of the caliper side the piston is sitting on for a rough guide to how deep the piston is...a very rough guide though.
how far can you allow them to extend before they pop out completely?
It should be safe to extend them the thickness of the friction material on a new pad. If you have some old pads, you could try chiseling the friction material off the backing plate and then just fitting the backing plate and pumping the lever (with the wheel fitted, obviously). This should extend the pistons without popping them right out.
Ime dot fluid gets filthy as the rubber diaphrams degrade.
I haven't used Guides or Codes, but I've never had problems with diaphragms deteriorating with Juicys or Hayes HFXs
key bit in the Marshy vid for those with XT and above with the freestroke adjusters is he does back the screw out before you bleed.
if you do this it def helps the bleed and then also actually means those screws actually do something compared to all those that say they do nothing - wonder if ready bleed from Shimano are perhaps done from the factory with the freestroke screws wound in...
My understanding of the freestroke adjuster is that it advances the piston, so the tranfer port is closed with less lever travel. If you advance the piston too far, it will partially close the transfer port and make bleeding more difficult. Backing the screws out should make sure the transfer port is fully open.
I suspect one of the main reasons Shimano brakes are tricky to bleed is the mineral oil is more viscous than DOT fluid, so that makes it more difficult for air bubbles to escape. This would explain why people report that using different brand mineral oil makes bleeding easier and also explain sensitivity to the freestroke adjustment.
Nnnnnnnggggg
Move bike/lever around to allow any trapped bubbles to escape from lever.
100% this. I was having a 'mare on my XT brakes until I watched a Park Tool tutorial on YouTube. Moving the lever released a surprising amount of air into the funnel. All good after that and no wandering bite point.
Yep it surprised me too.
