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Brake pressure


Lach's picture

By Lach - Posted on 25 June 2010

I'm losing pressure in my rear Shimano LX integrated brake system. After the bike has been idle for a while, I can pull the lever into the bars. A bit of pumping on the lever it brings the pressure back up and it works OK. Is this likely to be just a bleed issue, or are there other possibilities? Can't see any leaks anywhere.

As a further complication, I've also burred the screw head on one of the two little phillips heads that hold the cap on the reservoir, which makes bleeding or topping up with fluid difficult. Any thoughts on how best to replace that, or am I up for a whole new "brifter" mechanism?

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Rob's picture

Funny you mention this - I get the exact same issue with an Avid Juicy Ultimate front brake.

I've learned there's no point playing with it, because after riding for a few minutes it's all good but is a little disconcerting!

True, probably needs a bleed, maybe I shouldn't be so lazy? Eye-wink

hathill's picture

Easy and cheap to bleed, just need some mineral oil - at least rules out the easy things first! As for the screw, probably find one the right size in an old VCR or something similar.

Flynny's picture

Yeah your up for a bleed. To get the damage phillips head screws out you could carefully cut a slot in the head with a hacksaw to allow you to use a flat blade and then replace them once you have the out. If that doesn't work carefully knocking it around with a pin punch or drilling the head off the screw. Once the clamping force is relieved the screw itself should only be finger tight, just don't drip to far!

Rob, yeah stop being lazy. It's a 5min job.

Having that air in the break line effects the way the break work as the air expands more more than hydro fluid as it warms up. This makes the lever pump out the more you work the breaks and on a long down hill can lead to your breaks locking on.

ps's picture

however I recently had the front brake on my scooter touching the bars every morning. One or two pumps and it stayed solid all day. This turned out to be a vacuum issue in the reservoir. Once the reservoir cap was removed and replaced it was fixed with no need to bleed the pressure side of the system.

Antsonline's picture

...you go for the full bleed, you might want try the ole 'rubberband trick'.

get a rubber band, pull the lever back hard (so you cant turn the wheel) put the rubber band round the bars and lever tight enough so that it holds it in place.
Leave to stand overnight.

Remove rubberband. Voila.

If they're still cack after that - bleed 'em.

p.s. use a rubber band, and not a zip-tie. The lever needs to be able to 'move' against the band away from the bar as it releases any air.

hathill's picture

That's the problem with most MTB brakes, they are not able to allow fluid into the system when the level gets low due to the reservoir being closed. Girlock car master cyclinders use a flexible bellows which seals the hydraulic system on the fluid side keeping moisture and dirt out but is vented to the atmosphere on the other side to allow the pressure to be equal on both sides allowing fluid to enter the system as the pads wear. The Shimano levers I have seen have this rubber diaphram/seal but it's not vented to the outside world. I really don't get why so many people have issues with mtb brake bleeding, they are a simple system and as long as you are not leaking air in past a seal somewhere should be easy to take care of.

Lach's picture

for the various bits of advice. Will start with the low tech rubber band approach and see how we go from there.

muvro's picture

Shimano reservoir caps are vented as far as I know.

Hop fiend's picture

just added more fluid after taking off cap and getting air bubbles out of system by squeezing lever till bubbles stopped.

Brian's picture

Just remember if you add more fluid when the pads are worn at all then when you replace the brake pads you will find it comes out when pushing the pistons back.

hathill's picture
"Submitted by muvro on Fri, 25/06/2010 - 19:45.
Shimano reservoir caps are vented as far as I know."

You are absolutely right - I just looked at mine again closely.

Hop fiend's picture

pulled pads out and placed in yellow brake block supplied with bike to prevent that happening.

Lach's picture

FWIW, the rubber bands didn't work, but I only had a few thin ones, so probably not enough pressure?

Anyhoo, bought some mineral oil, loosened off the screw that hasn't been burred and managed to move the cap sideways enough to get some of it into the reservoir. All seems good, at least for the moment, but with this weather I've only done about 15 kms on one ride in the last 10 days and pulled up short on that as the front pads went down to the metal and have now been replaced.

If it hadn't been raining most of the day here, I'd head over to TH tomorrow for the listed ride to give it all a decent test, but it looks like a day for the "road" bike (aka Kona Dawg without knobbies).

Scottboy's picture

last weekend at Awaba when I was sweeping the course during the race , all seemed fine until yesterday when I pulled the wheels off to put in the missus car . I pulled up at my ride destination ( which is a secret spot ) turned the bike over & a pad was missing ,so I put a spare in took off on my ride & it was the same as the week b4 I looked down into the caliper the pad had fallen out . I took it to my mates place for a better look & noticed that on the piston the pin that holds the pad on is not there . There is the conclusion to 3 inside brake pads . 1 is at Coondoo the other at Awaba & the last at my secret spot

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