Changed my pads the other day .. didn't bother with a bleed as they had a solid feel before and after the swop.
On my first ride since I realized I had forgotten to do the standard "conditioning" runs so they lacked any bite.
I dragged the brakes on a fairly steep road descent and within 10 seconds or so the levers went from a solid feel to spongy and almost touching the bar ( considerably worse on the rear )
I couldn't believe they had faded that quickly and boiled the fluid so I stopped expecting to see a leak or similar but everything seemed ok and on setting off again they seemed to firm up again .
Later on the ride I approached another steep tarmac descent ( french country road) and the same thing happened but all was fine once the downhill had stopped . On the flat shortly after I could easily Endo or skid the rear with one finger braking .
What's causing this do you think ... Overheating ( surely not so quickly ?) or is it likely air bubble related and only becomes problematic when the brakes are dragged for any extended time on a descent ?
Can't really figure it out at the moment and I have no bleed kit . As stated ..the lever feels solid the rest of the time .
Ideas/help/solution would be great .
BTW ..the brakes are recent deores? (M6000 iirc) and I'm running 180/160 f+r as I mainly ride easy mixed terrain these days with a fair amount of tarmac for training .
Thanks in advance.
This is the famous Shimano wandering bite point. Bleed your brakes better.
Possibly and yes I suspect better bleeding may help but I cannot understand why the lever goes from firm to mush in 10 seconds of brake dragging ? Also ..normally dragging brakes continuously causes boiling sometimes and bubbles are the result but this can't be happening in such a short time surely ?. Why should a little air in a brake system cause such problems so quickly when it fine on the flat etc ?. Just like to understand !
Heat. You have heated the brakes up hugely thus a tiny air bubble has expanded massively becoming a big air bubble
Thanks TJ .. I had suspected that as a possibility but presumed in 10 secs it wouldn't get that hot inside the caliper . Off to attempt bleed without any kit! (Currently in rural France .. wonderful place ! )
Its only my best guess
Squirt some water on the caliper on the way down to cool it and see if that helps?
I'm not sure if it'll work but if as above it's heat in the caliper causing a bubble to expand it might tell you that.
Pull levers hard to bars and tie them off...leave them for a wee while (overnight). When you release them the levers will feel firmer. It has something to do with a slight pad reset but also having any air moving up to the lever reservoir, so if that does work, the bleed might just be a top up of the reservoir (if you have syringes, then a syringe at caliper with some fluid to help force the air out the reservoir would be better).
Thanks Dick .. yes I know that technique.
Despite the firm feel unless descending/dragging it appears there must have been air causing it as I removed the caliper to get it vertical and tapped everything whilst wiggling the lever etc. Just been for another ride involving steep roads and seems much better now.
If you didn't bleed it then you've moved the air to somewhere less 'obvious' so it should stay good for longer, but if you can get some more fluid in to push the air out that would be preferred.
My Shimano front brake (MT420) went like that. Did a bleed (pushing through from the bottom at the caliper) and was still bad. Fresh looking fluid was coming out of the lever end. I was confused about that.
Had to completely remove the fluid in the system through the caliper at the bottom and then push some clean fluid through downwards as some of the fluid was so dark grey it was almost black. After fresh fluid it were fine. Back brake still working fine and hasn't been touched.
If you've got the old pads still you can do a bodged bleed with them.
Take the wheels off, put the old pads in, pump the lever a few times to pump the pistons out, then push them back in with a flat screwdriver whilst the caliper is aligned so the hose exists upwards (i.e. air bubbles can escape up the hose). Repeat this a couple of times and it should flush any air from the caliper to the lever where you can remove it just by topping up the reservoir (will still need a funnel or syringe or something). Or as it's shimano it'll just sit there in the reservoir doing no harm as it's above the master cylinder.
Or as it’s shimano it’ll just sit there in the reservoir doing no harm as it’s above the master cylinder.
Air in the master cylinder reservoir can cause wandering bite point problems. If you lay the bike over, it can get into the master cylinder, especially after the pads wear down and there is less fluid in the reservoir. It's important to make sure the reservoir is filled and there isn't any air in there.