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Hi folks: just rebuilt front and rear master cylinders on an 81 MHR and I can't get the rear cylinder to prime with fluid. Initially, the new piston squeaked a little then quietened down. I've already pulled the banjo and still can't get the master to prime with fluid. I've never had this issue before so any advice would be most welcome.

Just did the front; primed immediately but took an age to get the bubbles out. Initially the front piston was smooth but now it feels rough. An easy job gone wrong; brand new pistons in front and rear. At least I have working front brakes but I don't like the feel of the lever at all. The last time I did this was front only on an old GT860 30 or more years ago; it was a breeze. What am I missing?

I used Motul 5.1 fluid

Is it possible totake the whole system off the bike ?
Then You could position it so that the air flows out on itself.
Later You can put a little pressure on the master, the bubbles get smaller and flow faster.

I made sure the "business-end" of the piston was VERY smooth, same for the lever and applying grease is a must.

ciao
ducadini

Sps : DOT 5 made my brakes a bit spongier, what is the Motul like ?

81mhr900ss wrote: ↑Sat Apr 20, 2019 12:06 pm ... and I can't get the rear cylinder to prime with fluid.

Not quite sure what you mean when you say 'prime'. I mean, normally all you have to do is to fill the reservoir, then action the lever so that fluid passes via the transfer port into the master cylinder and then down the line (with the displaced air coming out either at the bleed nipple(s) on the caliper(s) or via the m/c).

And repeat ...

That said, it does help if you initially 'prime' the m/c downstream of the piston by injecting a bit of fluid (using a syringe or whatever ...) into the end where the line connects.

81mhr900ss wrote: ↑Sat Apr 20, 2019 12:50 pm Initially the front piston was smooth but now it feels rough. <snip> At least I have working front brakes but I don't like the feel of the lever at all.

Ok, you need to be clearer. Is it the PISTON that feels rough, or is it the LEVER?

If the later, have you greased the pivot point where the lever mounts? And also the end of the lever where it acts on the piston? For this latter, it's best to use silicone, i.e./e.g. brake, grease.

If the lever is smooth, but the piston action feels rough, did you liberally lubricate the seals on the new piston with silicone grease before fitting it? And did you hone the bore of the m/c with some fine wet and dry? New seals can catch on even a minor irregularity or lip.

And they won't last, I'm afraid ...

HTH

Craig

ducadini: I used DOT 5.1 which is glycol based like DOT 3 and DOT 4. DOT5 is silicone based and should not be used in our Brembo systems.

Craig in France: Sorry for the lack of clarity. I think I worked out what's going on:

The front brake lever must have been knocked sometime over the last 27 years and the clamp part that the lever slides in to is not perfectly straight; the underside of the lever was scraping on the edge of the "clamp" causing the roughness. I pulled it apart, gently filed the clamp corner, added an extra washer on the bottom inside of the lever and now the lever is smoother. Yes, I had greased the seals with Brembo grease before reassembling the new pistons. I had also greased the pivots and piston ends.

The rear lever ended up being a problem because the tiny hole had re-plugged with something; I originally cleaned it with a guitar string; must have missed a tiny spec of something in the reservoir.

Both brakes now work but I suspect the front still has a bubble as the lever pulls within 1cm of the bar grip and is spongy. I'll re-bleed tomorrow.

Just pulled the heads, shimmed the valves; bleeding brakes is the easy stuff, or at least it should be.

Good - all solved!