Wierd Hydro disc issue !!

Discuss light weight issues concerning road bikes & parts.
Post Reply
Posts: 619
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

Have had some feedback from elsewhere that it's air in the system, but I'm not convinced.

Background :
- 2016 Cannondale Slate, RS505 levers & MT520 4 piston calipers (original metal unfinned pads)
- Approx 2,000km from new, 1,000km with me.
- RS5050 levers are original, MT520 calipers about 4 mths/400km old
- New disc rotors due to new wheelset (only about 100km use)
- all bled fine, and no issues whatsoever since I've been on the bike

Issue :
- Riding along gravel/single track path, with most of the 1st 17km uphill (450m ascent)
- get to the top of the trail and start heading down hill (-2% gradient to start with)
- couple of light braking applications, all good
- next application, front brake lever goes to bars with no braking at the front at all :shock: . Rear is fine.
- Couple of quick pumps, lever back to normal and all good (solid feel like before and not spongy at all).
- Stop and have a look - no leaks, no issues visible, caliper/disc not hot (or even warm), warped or loose.
- Keep riding and talking it easy. Both levers feel fine, and nothing out of the ordinary.
- Steeper gravel descent starts (-5 to -10% but straight) requiring a bit more braking, front lever goes soft again. :shock: :shock: One pump and all firm again :?: :?:
- Taking it real easy now, but occurs once more. That's a total of 3 times in about 2km. Each time after a 2/3 pumps, lever feel and capability returns to normal.
- During one of the 'soft lever' episodes, when pumping the lever, pads are observed to move as normal.
- During remainder of ride back to work (mostly on road), no further issue experienced and brakes functioned normally.

What the ???

Anyone with any idea/suggestions ?

Best I can come up with is that the pads over-retracted (they are partly worn, but still plenty of meat), hence the soft lever, and once pumped back up, all good. Discs run true with no rub.

Que ?

Posts: 2212
Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

I'm not a disc brake guy but it seems like the issue is inside the brake lever. Pity it's such a hassle to swap the hose left/right.

Out of your 1000km, when did the issue begin?

What if you are simply running low on brake fluid? Is there a way to top up the reservoir? I'd check shimano manuals and try to do so.

by Weenie

Posts: 421
Joined: Tue Jan 01, 2019 1:36 am

by TheRich

Make sure there are no leaks, especially if your bike has two-part brake lines (with the connector between the hose coming from the levers and the hose to the caliper)

Rebleed and then use something to pull the lever to the bar and leave it like that overnight.

User avatar
Posts: 833
Joined: Thu Mar 09, 2017 5:35 am

by Lewn777

Sounds like an air leak or there's an air bubble in the system, at the caliper or lever or just a low oil level. Firstly check that the front brake line is properly connected at both ends and that everything is torqued up to spec. Then try lifting the back and front wheels off the floor and check that the lever stays firm at those odd angles. As TheRich says if still no luck try a rebleed or a partial rebleed. If still no luck then there might be a bad seal or something and try a warranty.

Posts: 92
Joined: Sun Sep 23, 2012 1:53 pm

by numberSix

The simplest explanation is the reservoir was low, and now there’s some air in the line. Rebleed it, and fully fill the reservoir.

The goal is zero airspace, so plan on having a rag underneath as putting the cap on should cause excess fluid to squeeze out. Make sure the cap seal is in the neutral shape, not extended.

Posts: 619
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

I’m coming to the conclusion that the above comment from numberSix is probably the most likely, but can’t quite see how.

I have a similar setup on another bike (RS685 levers and See calipers) that I have been running for many years without this issue. And I’ve even worn a set of pads back to almost bare metal and no issue.

Current pads are still >50% wear left.

No leaks at all.

Any bleeds have been done with ethe right tools and Shimano funnel, and pushed fluid though from caliper to ensure no air.

Lever has great feel and is same before and after incident.

Haven’t yet had a chance to re-bleed due to family and work issues.

Only other thought is that reservoir size for RA-685 and RS-505 levers are different ?

by Weenie

Post Reply
  • Similar Topics
    Last post