Brake master cylinders

[Rockburner]

If there was a leak wouldn't we expect to see some signs of fluid escaping?

I've had poorly refurbished calipers in the past (refurbished by myself I hasten to add), that wouldn't go firm, but didn't leak any fluid. Refurbishing "properly" solved the issue. (finding the correct seals that go between the caliper halves).

[mangocrazy]

If there was a leak wouldn't we expect to see some signs of fluid escaping?

I've had poorly refurbished calipers in the past (refurbished by myself I hasten to add), that wouldn't go firm, but didn't leak any fluid. Refurbishing "properly" solved the issue. (finding the correct seals that go between the caliper halves).

Yes, it's a case of small amounts of air gaining ingress, not so much fluid leaking out. The seals/pistons would need to be pretty badly worn for leaks to be visible.

[iansoady]

But I'm not sure how air would get in but fluid wouldn't come out given that there is no differential pressure for the air to make its way in whereas there is in the opposite direction - at least when the brake is operated. Unless Maxwell's demons have been busy......

[mangocrazy]

Air molecules are substantially smaller than brake fluid molecules, so can pass barriers much more easily.

[iansoady]

True, but there has to be some force persuading them to do so.

[mangocrazy]

True, but there has to be some force persuading them to do so.

Suction, in the case of air. Pressure outwards is applied by your hand on the lever. When you release that pressure, suction takes over.

[roadster]

On a completely different tack beware of third party levers which don't allow the piston in the master cylinder to fully return. It is essential that the port which allows fluid to enter from the reservoir is open when the lever is released. I have seen (car) mechanics spend hours of head scratching because the pedal push rod was adjusted too long and removed all free play.

[Rockburner]

True, but there has to be some force persuading them to do so.

Suction, in the case of air. Pressure outwards is applied by your hand on the lever. When you release that pressure, suction takes over.

I'd imagine that the hygroscopic nature of brake fluid has an effect on this too. (even if small).

[Beancounter]

Somehow I now have properly working brakes where the lever doesn't come back to the bar when on the farthest setting - even on the mid-point setting I've got very good pressure. The annoying thing is I don't actually for certain know what change had the biggest effect. I tried:

Lots of bleeding both the conventional pump-pump-hold of the brake lever whilst opening the bleed nipple/s and using a Mityvac;
Changed the brake lever perch and master cylinder - this improved the problem but didn't fix it;
Fitted new pads all round;
Fitted new front seals;

Even with all of the above the front brake was still spongey, just not as bad. In frustration I decided to remove the ABS and fit two Hel braided lines at the front. The ABS unit sits in front of the swing arm and in between the frame so wasn't actually going anywhere. Maybe I managed to dislodge some air bubbles - who knows?