Brake issue

[RichieM]

Hi
Sorry to start my membership with a request for advice.
I've got a R100gs.
I was having front brake issues in that the front caliper wasn't releasing properly.
So far I have replaced the caliper seals and O ring, fitted a stainless brake line. Replaced the master cylinder piston etc. I've cleaned out the master cylinder and the fluid return hole which was blocked.
My problem now is that I can't get any pressure at the lever. I have primed and bled the system every which way. There are no leaks. I burped the master cylinder and there seems pressure there with my thumb blocking the outlet.The pads are moving but just no pressure and the lever is coming back to the bar. I'm sure there is no air in the system.
Any ideas

 

[Gelandestrasse]

Welcome, and don't worry about asking for advice first off - it's what the forum is for! You'll likely get a lot, with some of it conflicting, so here goes with my tuppence-worth!

Try ty-wrapping your brake lever to the grip and leave it for a day. Your symptoms are explainable by air in the system. A less likely cause is that your master cylinder piston is the wrong size or just does not seal.

Oh, and the cause of the original issue was most likely the blocked return hole, not the caliper. However, (less likely) any corrosion in the caliper piston bores will affect their movement and resist their release of the disc. Sorry of this is 'teaching to suck eggs'.

 

[Nin]

The Motocrosser bleed above is a good technique. Alternatively try goosing fluid up through the bleed point using a syringe.

But there is air in the system, if you have no fluid leaking.

 

[rocky burns]

Best way is to reverse bleed with a syringe. Make sure you have one big enough so you aren't having to try and refill it.

 

[Hatcho]

Another trick is to unmount the caliper from the fork and give it a few taps with a mallet while orienting it in different positions. This process allows the air to reach the bleed nipple and be ejected when bleeding. Sometimes air will be trapped in a newly rebuilt caliper and can't escape through the bleed nipple.

 

[RichieM]

Yep I have tried both of those to no avail.
I think the most likely culprit is the master cylinder despite the new seal kit.
I have taken the plunge and got a new one. I just need the time to fit it. I was speaking to an ex bmw mechanic who advised not to use full lever strokes when bleeding.

 

[RichieM]

I will also try the mallet trick. Thanks

 

[Mad Hatter]

Watching this with interest because I’m shortly going to tacking filling and bleeding the new caliper, brake line and master cylinder on my project bike. I’m anticipating similar issues

 

[SteveHop]

Tap the master cylinder with the back end of a screwdriver whilst operating the lever .
Worked for me

 

[RichieM]

Well
As it turns out the master cylinder was fine. If anyone wants a good 2nd hand one with new seals please contact me.
After much head scratching again and another bottle of fluid together with all the helpful suggestions I've got it 50% better. I used a piston spreader to give something solid to bleed against and with much tapping caliper removal and burping, another bit of air came out. I'm at a loss as to why it has been so complicated on such a simple system

 

[RichieM]

The only other thing I changed is to a braided hose. I can't think that would have effected it ?

 

[RichieM]

Finally sorted.
As it turns out, somehow the caliper was slightly misaligned on the fork.
2 shims behind the bottom bolt and 0 at the top.
And working fine.
How did that happen as there were no shims when I took it off.