Hello,
This morning I discovered a small spot of brake fluid on the floor, just underneath the rear master cylinder.
After some investigation, I found out that it comes from the screw that retains the cylinder piston.



The bolt was tightened with 7 Nm and for a reason or another it starts to leak.
Did I push the cylinder too deep (while applying brakes), damaging the screw thread and thus causing this leak?
Any suggestions how to solve this leak.
Are my brake pedal adjustments correct? I adjusted the screw in such way that it makes contact with the piston, without pressure on the piston. Or should I leave a gap of a few mm between piston and adjustment screw.
Need your advise.

Luk

It should be a aluminium gasket under the screw head.
If it's still there, it could be that it's have become flat and hard...
and you then need a new one.

Inge,
When I did the MC rebuild, the revision kit came with a new screw and aluminium gasket (which I installed).

Luk

Then a guess would be that the surfaces for the gasket haven't been clean or
not been undamaged....or the screw is cross threaded.

Inge,

I'll take the screw out and see. I'll have to bleed the brakes again I guess.
If there is damage to the threads, can I put some sealant (scilicones) on them?
I have a spare gasket, which I will install.

Luk

If the threads is damaged, I would have drilled and tapped new threads in
the next size (5 mm).

Inge,

Lucky me, the threads are not damaged.
MC has been repainted and as brake fluid has the capacity of eating paint. I guess that the paint dissolved, creating a small gap between gasket and retaining screw, causing the leak to become more important as the paint dissolves more.
I'll take out the MC, clean the surface between gasket and MC, install a new gasket, torque the screw with 7Nm and hopefully the leak will be cured.
If not, should I apply some sealant on the screw (something like Locktite fluid gasket)?
Don't like the idea of having to replace the whole MC just for a stupid bolt leaking.

Luk

I think it's gonna be a very small chance that it gonna leak when you have clean
surfaces and a new gasket.

If you afterall should be so unlucky .... Loctite would be OK, take care so you
don't get any of it inside the pump...and let it have full cure time before filling
any brake fluid.