I've got a set of M775 XT discs on both of my mountain bikes. One set I've never had a single problem with, the other set... Well! I've bled them many many times, and they are stupidly inconsistent...
Is there a sure fire fix?
I reverse bleed, with a large syringe, using proper Shimano Hydraulic Brake Oil, but it seems that though when I've finished bleeding them they may feel really good, wait til I get the bike out on the trails and by the time I've ridden an hour or so, every time I go to grab the brake there will be nothing, unless I pump the lever several times! Yesterday down the Forest of Dean, going into the final swoopy descent on the Blue Verderers trail, I had no back brake whatsoever. I was just glad I've ridden it a few times before, and know what to expect, cos it was a challenge not being able to use a rear brake at all.
air in them - perhaps too low fluid level in the master cylinder as well. Or blown master cylinder seals
TJ, I've bled and bled them, there shouldn't be any air in there at all, though I appreciate obviously there is some getting in there otherwise they wouldn't feel like they do...
Fluid level in the master cylinder is as full as I can get it after bleeding, I know the consequences of leaving it too low so make sure it's always as full as can be.
Assuming you're bleeding them correctly, which it sounds like you know how to, I'm guessing you've got blown seals somewhere, as TJ says. Brakes don't, or rather shouldn't need constant re-bleeding, if the system is air-tight.
Are both brakes the same, or is it just the rear one that's dodgy?
Try pushing the pistons right back in before bleeding - sometimes bubbles of air can sit behind them. try tapping the calipers
Make sure the bleed port is the lowest part of the system if reverse bleeding them - take the caliper off the bike if need be
Could be a poorly fitted/damaged olive. Have you shortened the hoses at any point? Did you use new olives? I'd be looking to strip the hoses off and reconnect them all with new seals/olives and making sure there was no crap in any of the joins.
Cheers TJ, will try that
I bought the brakes 2nd hand, so never shortened the hose and fitted the olives myself. Though given I've probably got a few cm to play with on the hose length, and new olives aren't expensive, it might be worth a go druidh.
And it's just the rear that's dodgy. The front isn't working as well as it should be (though I think that's down to pad contamination, lever feel is as it should be really), but it's the rear that keeps pulling into the bars. Like I say though, never had this issue on the same brakes on my other bike, though I have owned those from brand new.
How could I tell if the master cylinder seal had blown? And is this a replaceable part?
No seals available for shimano brakes.
I had one rear brake that I struggled with for a while. I thought I was bleeding it OK, but it was never "right". I eventually took it off the bike, hung the while assembly vertically (lever to the top) and just get tapping the hose a few times every time I passed it. Thereafter, it was just fine. Seems there was just some air in there that was determined not to come out.
I've been having the same issue as you mboy, but with the rear on my M785 (which I've put goodridge hoses on). Feels firm when first bled, but after an hour or two of riding (but not if the bike isn't used) the lever comes to the bars. It's ruined a couple of trips to Afan & the mendips recently. Did the same as druidh for the last bleed & am off for a weekend at Brechfa/Doethi on friday, so hopefully its solved it.
Bleed it. Pull the brake lever and tie back with a zip tie or rubber band overnight. Check for oil around the point where the hose goes into the caliper. If you can see fluid, the problem is with the hose so it's back to where you bought it from time.
freestoke was the issue with mine, all was fine until you'd been riding a while.
did require a bleed as well though.
They wont bleed properly with freestoke either in or out, cant remember which. I had same problem and Skyline Cycles mechanic changed freestroke and bled no problem.
Before you spend any money on parts, do a syringe bleed again, but beforehand put a couple of 2 pences in the brake then pump the cylinders right out. A big load of bubbles came out of mine when i did this and it fixed the problem. I'd done about 5 bleeds before hand with no improvement.
scruff - Member
They wont bleed properly with freestoke either in or out, cant remember which. I had same problem and Skyline Cycles mechanic changed freestroke and bled no problem.
Right - would be intererested to know whether it's open or shut - that might be my problem
If you have two sets then change the diaphragms around to rule them out.
Like Retro I bleed mine with worn pads and thick washer before seperating the pistons to fit the yellow spacer. This normally expels a bit of fluid as you seperate the pistons so you know they maximum fluid in the system then.
Bleed it. Pull the brake lever and tie back with a zip tie or rubber band overnight.
Do this. I think it basically encourages any air bubbles to find their way up into the reservoir. I find if you do this religiously before every ride, it'll fix a lot of problems. Bit of a faff, but it works. Try leaving it overnight as described and then bleeding from the bottom up, see if that helps force some of the bubbles out.
Gonna follow the advice and push pistons all the way out first to try and expel any trapped air bubbles. All the other tips mentioned, have already tried them all before.
Now the only issue I have is I can't get the bloody Phillips head screw out of the lever holding the master cylinder on. Previous owner seemingly rounded the head off slightly, and now I can't get it back out again! Gonna be a pliers job probably, possibly even a hacksaw and flat blade screwdriver, then a new screw!
Arse
Looks like whichever bike I want to ride this weekend, I'm going to have to rob the other for parts temporarily to make it work, unless I go out on the road bike!
Should have bought Hope, you would have them fixed by now for 5p or free!!
The 775s can be a real pain in the hole to get right. The two problems you will hear about are trapped air and blown seals. I used to be able to revive mine with bleeding and a cable-tie on the lever. Eventually the seals went. I haven't had any problems with the SLXs, I top up the fluid as soon they feel soft and they work great.
I thought Shimano brakes never failed? Only Hope brakes dont work, right?