Just changed the fluid on my Shimano M965 dual control brakes and can't get the reservoir to seal on either brake. Have bleed them loads of times before. Basically when I screw it back down and depress the lever it bubbles where the excess fluid is around the cap.

It done this before but unscrewing topping up and retightening always sorts it out. This time I've pretty much used half a mineral oil bottle undoing, topping up and retightening.

The rubber insert look fine and wiped and rewiped incase of grit. Just can't work out what issue is.

I have just fitted some longer forks and wondered if reservoir maybe wasn't parralell with the ground so fixed that but still.

I wondered if last time I did it I might have overtightened the screws as they were really hard to get off this time?

Realy appreciate any ideas.

not the breather hole for the air on top of the diaphram to get out when expanding bubbling air into the excess fluid is it ?

Hmmmm might be. So when the oil gets pressurised and unpressurised it breathes air moves in and out?

Doesn't even mention that on the park guide.

Thanks for your help!