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Stiff hydraulic lever and slow-to-return pistons

by part_robot
Sat, Jul 15, 2017 11:30
10

Hi all,

The rear lever on my new ST-RS685/BR-RS805 hydraulic brakes is quite stiff and the calliper takes about 0.5-1 second to return. I've bled the system and lubricated the calliper and yet it still has this issue. Meanwhile the front brake is perfect.

I'm thinking about returning it but before I do I wonder if anyone's had similar experiences and if so how they fixed them? I'd rather fix at home than go through the rigmoral of removing all the components and waiting for a replacement.

If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.

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10 comments

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part_robot | 7 years ago
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The answer (just resurrecting this old thread) was; flow  slightly restricted due to a burr where the hose was cut. Removed burr and everything was fine.

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StraelGuy | 7 years ago
0 likes

Good stuff, thanks, will get the kit. Did you have any issue with the length of the bolts for mounting the caliper? I've read that is depends entirely on the effective depth of your chain stay but that the kit only comes with one length?

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StraelGuy | 7 years ago
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Interesting, glad you got it sorted. I've literally just ordered the exact same brakes, any hints and tips for fitting and bleeding them?

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part_robot replied to StraelGuy | 7 years ago
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guyrwood wrote:

Interesting, glad you got it sorted. I've literally just ordered the exact same brakes, any hints and tips for fitting and bleeding them?

 

Step 1: buy the kit from here:  http://www.epicbleedsolutions.com/

 

Step 2: follow youtube videos. But in summary:  open bleed port on shifter and thread in the threaded syringe  (A) (without the plunger). Full that syringe half way up. Attach syringe (B) and hose to calliper bleed port and pull down a little bit of a vaccum with it. Open up the calliper bleed port and pump the brake; the fluid will be pumped from the top syringe at the shifter to the bottom syringe at the calliper. Keep going, replacing the fluid at syringe A  as needed. Throughout, tap the hose, shifter and calliper to encourage more air out. Once done, close the calliper bleed valve and pull down a vaccum on syringe B so it slurps up the fluid as you pull it aray from the port. At the calliper end use synring B to empty out the fluid that remains in syringe A. Unscrew syringe A and wipe away fluid. Screw in the bleed screw. Done.

I recommend doing this with the wheels off, pads out and callipers detached from the frame; that way you can angle it all around to ensure the last bubbles are removed.

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part_robot | 7 years ago
2 likes

For anyone following this, it turned out to be a tiny piece of swarf in one of the brass fittings that are pressed into the hose.

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CXR94Di2 | 7 years ago
0 likes

It just might be just stiff o rings seals on the caliper,  if it doesnt bind I wouldnt be too worried.  

Aside, Hope do RX4 calipers with 4 pots and bigger pads.  Im thinking of a winter upgrade to my Shimano hydraulic setup

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part_robot | 7 years ago
0 likes

Thanks. Good advice!

However, there's no air on this system; I'm absolutely certain of it. Like you I also do various tricks to get the air out including removing the callipers and putting them at an angle that moves the air to the bleed hole as well as suspending the frame as an angle to get the air past the bottom bracket.

The lever isn't soft - the brake responds immediately and in the same way as the other lever. It also - unlike if it had air in it - bites hard without sponginess beyond what you'd expect from the designed elasticity of the system. The issue is purely that it's stiff and returns slowly.

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CXR94Di2 | 7 years ago
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Try bleeding again or leave the filling cup attached and put an elastic band on the brake lever to pull it on.  This will open up the reserviour to let air(if any) escape.  Leave it over night to complete.  

I prefer to inject fluid from the caliper end to force air upwards.  Being oil sometimes it can be  slow for air to escape.

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part_robot | 7 years ago
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They move out some amount for sure and I can pump them out a good few mm with a few pulls - sufficiently to give me enough exposed cylindrical area to get some brake oil around the pistons. The difference between them and the fronts is that the fronts move out under pressure and instantly retreat about half a mm when I let go, but the rears retreat the same distance much slower (as mentioned above it takes about a second). Due to the slow retreet if I have the rotors in subsequence pumps of the lever just ram them harder against the rotors and the levers become stiffer earlier.

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CXR94Di2 | 7 years ago
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If you take pads out, gently press the lever, do pistons move out freely 2-3mm?

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