So my pads are about 50/75% worn but I noticed after I leant the bike over on a bank to fix a puncture that the lever for the rear brake depressed all the way to the bars the first time I squeezed the brake and needed "pumping" to get it back to pressure - the front brake was fine though and the rear seems OK now.
Anyway, what should I do here:
Top up the fluid in the reservoir - if I do this will I then need to remove some oil when I replace the pads??
Bleed the system fully/replace the oil - am I likely to now have air in the system although the levers don't feel spongy.
Replace the pads.
All or some of the above?
Something else/nothing - was the initial pumping needed because I perhaps inadvertentlypushed the pads/pistons back a little while refitting the wheel - which actually, as I type this does seem like quite a plausible explanation....
The brakes are MT420 4 pot and the levers are BL-MT401 (there is no adjustment available on the lever itself.)
TIA 🙂
Do a mini bleed. I do them fairly often and never have a problem fitting new pads.
https://epicbleedsolutions.com/blogs/guides/5-minute-shimano-mini-bleed
If it were me, I'd replace the rear pads anyway.
The system is probably underfilled and has air in the reservoir. I would fill both reservoirs with the worn pads in place. This will overfill the system. You'll need the funnel that fits into the bleed port in the lever. Just fit that and put some fluid in, then pump the levers for a minute or so. This should let the air bubbles escape out the bleed port and fill the reservoir. It will now be overfilled, so you will have to remove the excess fluid in order to fit new pads.
Before you fit new pads, open the bleed port on the reservoir (with the lever turned so the bleeder is at the highest point in the system) and lever the old pads apart with a flat bladed screwdriver to push the pistons back into the calipers. This will force excess fluid out of the reservoir and should leave the system full and without any air. Then close the bleed port on the reservoir and fit the new pads.
Personally I'd start again. Take the brake off the bike, take out the pads and pump out the pistons one at a time. Clean with some alcohol then lube using hunters silicon lube, then go through the process of recharging the system.
Fit, line up, tap this way and that and bed the new pads in.
If you dont want to take off the brake, leave it on, only it makes servicing more tricky. But cleaning up the pistons, which have obviously been sticking out further given the pads are worn,and will have grime and grit stuck to them, which can only damage the seals when you force them back in. Solve problems before they become problems.
Regular service is important, especially given the importance of brakes.
Yes, it makes sense to clean up the pistons while you're at it.
Thanks Peeps
Clean with some alcohol then lube using hunters silicon lube
Can I lube with the mineral oil? I have seen this mentioned on a couple of YT vids..
Can I lube with the mineral oil?
Lubing it with the correct fluid for the brake system is a safe option. When you push the pistons in, only a very thin coating will stay on the piston anyway, the rest will just be wiped off. When the piston extends again, it will be coated in brake fluid from within the caliper, so most of the lubrication will come from the fluid, not anything you apply externally.
Ta - if I have already pushed the pistons in without cleaning them will I have knacked the seals....? They won't have been really mucky as I clean the brakes with water/compressed air regularly.
They should be fine...if I'm being very lazy (despite what my last brake bleeding thread sounded like), I clamp the levers closed overnight, then release them, connect a syringe up with the caliper and remove the bleed nipple from the lever and just push some fresh fluid in from caliper...the air or space in the system will be removed via the lever and once topped up, you replace the bleed nipple and remove syringe from caliper.
That doesn't sort a full bleed but I also appear to be a bit strange as I don't like a lot of lever pull on my brakes, so when doing a full bleed, I don't push the pistons all the way back in, that way there is less lever pull (does mean more chance of brakes rubbing though.
I am missing something here. The brakes feel fine, and OP had to pump the lever a few times after pushing the pads back?
Don't touch them, you'll only make them worse!
If you top the fluid up, when you push the pistons back to replace pads later the fluid will overflow at the master cylinder or cause the brakes to stick on when they heat up if the system is not vented. Fluid level dropping is a normal result of pads wearing, topping up is not a bad thing, just be aware of the possible consequences.
I am missing something here. The brakes feel fine, and OP had to pump the lever a few times after pushing the pads back?
The system is underfilled and probably has air in the reservoir. When he leaned the bike over, the air got into the master cylinder. Holding the bike upright and pumping the brakes worked fluid back into the master cylinder and let the air back into the reservoir. The fix for this is to properly fill the reservoir so that it does not have air in it. Seems to be a very common problem with Shimano brakes, hence the wandering bit point complaints.
Thanks all - had a total nightmare doing my first ever bleed! Learned some valuable lessons though. Did rear first and despite getting fluid all over the place arsing about with the syringe it went pretty well. I have got a much better way of getting the syringe on now though. Top tip. Put a bit of fluid in whatever you have attached at the brake lever end and then pull the syringe as you open the nipple. All the air still in the syringe pipe goes up to the syringe cylinder and any fluid that comes out is replaced at the lever end by the bit in the pot. 😀
font brake... oh dear. Managed to push one of the pistons out while cleaning them up. Fluid all over the place. I did get it back in and then spent a good hour trying to get all the air out of everywhere but I think I managed it. The front feels fine now and is full of fresh Shimano fluid ( no idea what was in there originally but it was clear, well, clear and dirty. Definitely not red. It was whatever was in the bike from new last July). It doesn’t appear to be leaking but Ill keep an eye on it. New pads on the rear and bedded in nicely so hopefully all good.......
Sounds like you had a good learning experience!
All the air still in the syringe pipe goes up to the syringe cylinder
Before attaching the syringe, hold it with the hose facing upwards and gently push until fluid is right at the tip of the hose. Pulling on the syringe when you attach it is still a good idea, but it should just be a small air bubble rather than "all the air".
The "right" solution to your problem is to bleed the system with a bleed block in it. Before closing the bleed nipple make sure you push fluid into the system to fill the reservoir. This *should* give you the right amount of fluid to compensate for pad wear, whilst still allowing you to get new pads in.
In practice, it's extremely hard to get a perfect bleed and as the fluid level drops due to pad wear, the inevitable bubbles will start finding their way into the m/c more frequently giving the wandering bite point that Shimano are famous for. You can put more fluid in to compensate for pad wear, but it'll need to come out again when you fit new pads.
as the fluid level drops due to pad wear, the inevitable bubbles will start finding their way into the m/c more frequently
If the reservoir is full and doesn't have any air in it, then there is no air to get into the master cylinder. If you bleed them with worn pads so the system is overfilled, then use a flat screwdriver to lever the pads apart and push the pistons back in, this will ensure the reservoir is full and doesn't have any air. Then you can fit your good pads.
They are meant to have air in .. it floats up to the reservoir then sits on top and doesn't affect braking .. if its topped ALL the way up to the funnel it's overfilled.
HOWEVER ...
Current global shimano fluid shortage aside it's always worth a bleed when you can.
Sooner or later the "black crap" sneaks behind piston seals and causes sticking (especially the master cylinder in the lever) and this is a PITA to repair... and unless you value your time at £1 an hour or like messing with fiddly shit not economic.
I've done it purely out of stubbornness and because I'm a whimp who rather fettle than get wet every day in winter... (usually I'd rather ride but cold and dark winter evenings when its pissing it down rebuilding a lever can see an interesting distraction) but I'd already replaced the lever for £25 or so...
They are meant to have air in .. it floats up to the reservoir then sits on top and doesn’t affect braking .. if its topped ALL the way up to the funnel it’s overfilled.
Nope, there should be no air in the reservoir. They have a rubber bladder that sits on top of the fluid. That provides for changes in the fluid level in the reservoir as the pads wear and separates the fluid from the air. If you fill them completely with the pistons fully retracted into the calipers, they are not overfilled.
They have a rubber bladder that sits on top of the fluid.
If you fill them so the fluid sits on top of the bladder though (which is easily done) they are over filled...
If you try and force the pistons back without releasing a bleed nipple you get problems (depends on actual brake, the newer ones have a port squirts it out, scared the hell out of me first time)
(Based on the experience of having a kid doesn't check his pads before riding so needed to replace trailside)
If you fill them so the fluid sits on top of the bladder though (which is easily done) they are over filled…
Fluid cannot get on top of the bladder unless you have really screwed something up. That bladder is a seal, with fluid on one side and air on the other.
If you try and force the pistons back without releasing a bleed nipple you get problems (depends on actual brake, the newer ones have a port squirts it out, scared the hell out of me first time)
The reservoir should be completely full with the pistons fully retracted. As the pads wear, fluid from the reservoir moves into the system, the fluid level in the reservoir drops, and the bladder extends downwards to stay on top of the fluid. When you replace worn pads with new pads, you have to push the pistons back into the calipers. This forces fluid back into the reservoir. If they were correctly filled in the first place, there will be exactly enough space in the reservoir to take that fluid. If there isn't enough space in the reservoir, the system is overfilled and you need to release some fluid.
If there isn’t enough space in the reservoir, the system is overfilled and you need to release some fluid.
From what you wrote it makes it sound like its impossible to overfill as fluid should be dripping out of the bleed screw.
I'm trying to point out as simply as possible that you can overfill especially if you do mini bleeds and the bleed screw hole above the reservoir should have air in.
It's impossible to overfill if you have the pistons fully retracted into the calipers. If you do it with worn pads, you absolutely need to push the pistons back into the caliper to avoid overfilling. With the pistons fully retracted into the caliper, you do not want any air in the reservoir.
It is impossible to get fluid on top of the bladder unless you have really screwed up. That bladder is a seal that separates the fluid from air. If fluid is getting past the seal, then something is wrong.
Before attaching the syringe, hold it with the hose facing upwards and gently push until fluid is right at the tip of the hose. Pulling on the syringe when you attach it is still a good idea, but it should just be a small air bubble rather than “all the air”.
Yes I did that - I'm talking about the tiniest bubble/s that inevitably form when attaching the syringe tube. On my first go I spent 10 mins tapping the tube to get all the tiny bubbles to go to the top before I opened the bleed nipple.
The “right” solution to your problem is to bleed the system with a bleed block in it. Before closing the bleed nipple make sure you push fluid into the system to fill the reservoir. This *should* give you the right amount of fluid to compensate for pad wear, whilst still allowing you to get new pads in.
Yes I had a bleed block in too. I also made sure that the reservoir was full to the brim before putting the screw back in. The bleed kit I bought had a syringe for the lever but no method of stopping whatever amount of fluid (albeit small after I syringed it out was left in it, from pissing out when I disconnected it, the Shimano pot system seems infinitely less messy.
I have managed to find a litre and a half of Shimano fluid for sale so I've ordered that (if it ever turns up) along with the Shimano pot thingy for the lever.
Next time it will be prefect and zero spillage! 😉
Pads is my next issue - no Shimano D03S pads in stock anywhere at all....
no Shimano D03S pads in stock anywhere at all
I think these are the right ones.
https://www.chainreactioncycles.com/nukeproof-shimano-saint-zee-trp-quadiem-slate-pads/rp-prod129817
thanks @thols2
Are the Nukes any good?
My LBS has some Aztec ones - they seem quite cheap (£6 a pair) so are they rubbish?