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Got an ongoing problem with an old slx rear brake that won't allow me to keep a solid feeling at the lever despite several bleeds . If I leave it with a strap on the lever overnight it stiffens things up but quickly gets soggy again and almost touches the bar if pulled fairly hard. If I pump the lever a bit it also stiffens up but again only for a very short time . I find myself doing this on any reasonable descent to get adequate power. Do you think it still has air in there somewhere or that perhaps a seal in the master cylinder is a bit weak so doesn't hold pressure well and hence pumping helps it build power again. I believe if that's the case no spares are available? The front is solid as a rock but obviously does have a shorter hose. Any ideas much appreciated. cheers.
Do you think it still has air in there somewhere
Sounds like it. You might get a better result if you can remove the whole brake from the bike (or have the bars high enough that the brake is vertically below them). I assume you are following the Shimano bleeding instructions?
When you get a hard lever and hold it on hard does it slowly come back to the bar - failed seals - or does it stay hard once pumped up? air in the lines
sounds like air to me
As Scotroutes.
Assuming the hose is in good order and there’s no leaks at the olive or banjo, the most likely answer is that the hose routing is allowing air to remain.
Ideally, having the whole brake off the bike and hanging down will make it easier to fix this, but can be awkward. If you have an old set of bars around, clamping one end of them in a workstand and fitting the lever to them allowing the hose/calliper to hang makes life simpler.
Alternatively, if you really want to keep the brake on the bike go to town tapping the hose while the bleed nipple is open to try to dislodge the air. Even just popping the brake off the bike from the chainstay can make a huge difference though.
Did you reset everything before the bleed? Pushed pistons back in and reset the lever reach and free stroke.
If you can't remove it from the bike remove the caliper so you can drop it lower.
thanks guys ..I have tried a bleed and been tapping the hose and caliper. I also hung the bike upright overnight hoping that would sort things by allowing air to filter it's way up but to no avail. once the lever is pumped solid it doesn't slowly come back to the bar if you hold it ..it remains solid . 2 questions though
1/..why does tying a lever back overnight stiffen things up only
-
temporarily
..I thought the idea was that the air could work it's way out?
2/ if you remove the whole shebang and hang it from the master cylinder I presume actual bleeding via the nipple is unnecessary ( if already done ) because the idea is that any remaining air can ascend up to the master cylinder?
I pushed the pistons back in and spaced them back before bleeding ...didn't reset the knurled adjuster ( free play ? )though iirc. maybe my bleed technique needs revising...I just attach a bit of tubing to the nipple then pump the lever till hard ...undo the nipple , pump again once or twice then close the nipple. I then pump and tickle the lever till hard. Always worked ok on other brakes incl various motorcycles.
Close the nipple before releasing the brake.
yes I do that!
the video was good but I have the older style reservoir and you can't attach a bucket so to keep filling whilst moving the caliper about may require some dexterity!Will give it a go though.
Possibly caliper seals. Work them in and out and lube up
1/..why does tying a lever back overnight stiffen things up only temporarily
..I thought the idea was that the air could work it’s way out?
No, the opposite. Tying the lever back will shut off the reservoir and any air in the line will definitely be stuck there. The tying the lever trick is generally useful to get pistons to advance if the seals are sticking.
if you remove the whole shebang and hang it from the master cylinder I presume actual bleeding via the nipple is unnecessary ( if already done ) because the idea is that any remaining air can ascend up to the master cylinder?
You'll still need to push fluid through the system to get the bubbles out. I've found that when pushing fluid in, giving the syringe a few sharp pushes can dislodge bubbles that don't come out if you're just pushing fluid gently through.
No, the opposite. Tying the lever back will shut off the reservoir and any air in the line will definitely be stuck there. The tying the lever trick is generally useful to get pistons to advance if the seals are sticking.
think you're wrong there ...even epic bleed solutions mention doing it if you get an air lock caused by the bike being upside down . Also it definitely has an effect at firm up the lever feel and reducing sponginess ..at least for a short time .
tying the lever back allows any bubbles to get to the top of the line - they will not get into the master cylinder while the lever is pulled - they cannot a the port is closed. When you release the lever fluid travels back into the master cylinder and that movement may take the bubble into the master cylinder. Its main effect tho is to reset stuck pistons.
It does not solve any air in the systema the air is still there in the master cylinder waiting to get into the lines and cause a spongy lever again when it does so. Bled properly and correctly there is no air in any part of the system including the master cylinder
[i]maybe my bleed technique needs revising…I just attach a bit of tubing to the nipple then pump the lever till hard …undo the nipple , pump again once or twice then close the nipple. I then pump and tickle the lever till hard. Always worked ok on other brakes incl various motorcycles.[/i]
I do not have encyclopedic knowledge of brake bleeding, but that doesn't sound right. For older Hope brakes, it is open nipple, squeeze lever slowly to bars, tighten nipple, slowly release lever, repeat, then top up reservoir, repeat...errr...repeatedly until no air appears at nipple. For Shimano, I thought it was best to reverse bleed from the caliper with syringes and the appropriate tubing/adaptors.
Reverse bleed also works well as air rises. Attach syringe to caliper, open nipple at lever and attach a receptacle (ideally another syringe). Hard push of plunger and allow top syringe to fill. Air goes to top of that and out of system. Can push fluid back in and repeat a couple of times - push back slowly so any air doesn't get back in.
think you’re wrong there …even epic bleed solutions mention doing it if you get an air lock caused by the bike being upside down
Pulling the lever shuts off the reservoir in the lever from the rest of the system, so it's going to prevent air from getting from hose to reservoir or vice versa.
Tying it back for an extended period can push the pistons through the seals causing them to sit closer to the disc, which will effectively raise the bite point, which might be what you're feeling.
Epic recommend tying the lever as part of a bleed. This is so that you can then pull on the syringe at the caliper and lower the pressure in the hose and caliper, causing bubbles to form and flow out.
I had an SLX rear brake that had the same issue. It was air in the caliper. It drove me mad though. I finally I managed to suck an air bubble out by pulling on the syringe rather than pushing oil in. Attached a syringe full of oil to the caliper, undid the valve and pushed some oil through to the lever. I then pulled back on the syringe and a bubble appeared!
Worth a try and make sure the pads are pushed fully in first
Great stuff ..you learn a lot on here...all the contributions much appreciated.
A few actions that always seem to get stubborn bubbles out:
Rotating the reservoir/lever through different angles - straight up, then 45 degrees from horizontal in each direction, then flicking the lever a few times at each position.
Opening and closing the bleed port a few times in succession, or even pulling back on the syringe very gently a few times as you're driving fresh mineral oil in.
the idea of tying the leaver back does 2 things. The first is it can allow the seals to slip on the pistons a little meaning they stay advanced better. this is often like pumping the brakes up. The other thing it does is put pressure on the whole system. This can cause tiny bubbles in the system to group together and make it easier to bleed them.
Must say I didn't realize there should be no air in the MC .Makes sense of the bladder now.I'd presumed it was the same as motorcycle MC's where you have an air space above the fluid causing no troubles .Should learn to read instructions I guess !
Well ..I tried the technique shown in the video posted further up the thread which involves removing the bleed nipple completely and think it has done the job so a good result and lots learned..cheers.
replace it with an Avid BB7 and never have that problem again 🙂