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My Bleeding Story

New (well, new to me) bike arrived so needed to give it the once over. Got round to the brakes, a squeeze on front brake sound, rear brake spongy. Pulled the pads out and found all 4 pads were well past their sell by date. New pads ordered so I thought what the hell, let’s give them a bleed. Bleeding is not new to me, in fact I cut my bleeding teeth with Avids OMG what a pain they are!! Then after that I had some magura’s, slightly better. Now I have Shimano’s on all my bikes so I was more than happy to see my new brakes are XT’s, not used to having the free stroke adjustment to tend with but google was called on. This got a bit confusing as there are some people who wind the screw all the way in, then some that say wind it out, then some that say leave it alone, and then some people who say wind in, then wind out during the bleed. Decided to leave it be.
First issue, with pads out I always put the bleed block in, so went to push the pistons in and they wouldn’t budge, no way the block was going to fit in. A small squeeze on the brakes showed that the pistons were moving so the issue was elsewhere. Realised that they might move in with the freestroke screw screwed out. Yes success, bleed block in, front brake bleed in progress. All I can say is what was coming out into the funnel can only be described as Satan’s snot. It was green, cloudy in parts, black in parts. All I know is that was no Shimano mineral oil in there. Anyway job done, took a while and lots of fluid to flush it out but end result was a great bite on the brakes. Onto the back spongy one. Once again, screw unwound to get the brake block in and onto new fluid, once again, beelzebub had blown his nose in the brake and took another big amount on sweet red Shimano nectar to flush out the horror show that was in there. Job done or so I thought, still spongy. So started again, pushed fluid up, drained fluid down, up down, up down, bubble bubble bubble, I carried on until no more bubbles. Job done, nope still spongy. Head scratched, YouTube visited, I followed it all. Luckily my father in law had been watching the whole process and chipped in, ‘I used to have a mini that was a pig to bleed it’s brakes, until my mate picked it up by the front with a crane to get all the bubbles out’ so a shift of the caliper up, down, left, right, out popped 2 big bubbles from the caliper. Winner!!
Hope this helps people. And yes I still love Shimano bleeds, I’d take them over Avids any day!
Onto getting the shifter cable internally routed, a whole new story!


  • steve_sordysteve_sordy Posts: 2,432
    I have done the standing the bike up trick a few times and it works.

    The theory is that unlike the front brake hose which is pretty much vertical, the rear brake hose undulates up and down and has loads of places where bubbles can get trapped. So it needs special treatment to ensure you can get all those bubbles out.

    Stand the bike on the rear wheel and tilt the bars so that the rear hose has a pretty much uninterrupted upwards path. Lean the bike against something (I use my workstand) and ensure it can't fall during the night. Secure the brake lever with a strong rubber band or a zip tie (my preference). This does two things: 1) By locking the rear wheel it helps keep the bike upright. 2) By keeping the pressure on, it compresses the bubbles and helps them to migrate upwards through the brake fluid. Leave the bike overnight.

    I then do a top bleed only, as all the bubbles are in the lever reservoir.
  • mully79mully79 Posts: 903
    The green snot brake fluid is probably Comma LHM+
    Seems slightly thicker than shimano mineral oil, half the price and available from Halfords. I actually prefer it after being forced to use it in an emergency. No idea if it will kill my seals eventually but i'm a long way past caring.
  • I don't know about Comma LHM+, but if it isn't mineral oil there is a very good chance that it will kill the seals, or have the wrong boiling point or something undesirable.

    But then I may just have been brainwashed by Shimano! :*
  • veronese68veronese68 Posts: 26,708 Lives Here
    Brainwashed by Shimano I'm afraid. Mineral oil is just LHM with a different colour dye.
  • reaperactualreaperactual Posts: 1,179
    edited September 2020
    Had rear brake bite point, lever travel issues on my SLX's for while, nothing extreme, just irritating. Been doing a lot of research on various causes and reasons behind it.

    Intrigued by a thread somewhere about Pentoline mineral oil being less viscous than Shimano's which supposedly helps with getting more air bubbles out, also lets fluid pass thorough compensator and return ports easier (especially in cold temperature) and lots of people use this stuff and suggests it cures these issues I have been dealing with.

    I've always used genuine Shimano mineral oil (same price as others so why not stick with it?) so was reluctant about giving this Pentoline stuff a try.

    Before that, I decided to do yet another last bleed or should I call it a 'super bleed'. You name a trick and I did them all, in one go.

    A few bubbles out of the lever, surprisingly non out of the caliper with automotive style quick open and close of the bleed nipple where I guessed there may have been some trapped air.

    This 'super bleed' has improved the rear lever travel and made general feel better than it's been before, still sceptical for now but time will tell?

    Now onto the front brake to see if I can eek any improvement out of that that one too!
  • I always unscrew the free stroke adjustment, as if its fully open then theres nowhere for air to hide.

    I also take my brakes off to bleed them. i then hang them from a workstand so caliper is hanging dead straight down.
    I dont follow convention with MTB bleeding i always use the moto style, open bleed screw, pull lever, close bleed screw, let go of lever.

    Just something thats carried over from my motorbike days.
    One other thing i do is leave the bleed port on the caliper off overnight with a zip tie holding the lever back, seems ot get rid of any stubborn bubbles.

    Always worked a treat for me.
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