After bleeding brakes on various cars over the decades, I was surprised to discover the hard way just how different bleeding bicycle brakes are. The first culprit is that the inside hose diameter is very small. With Shimano, they decided to make the newer BH90 hose inside diameter smaller than the previous BH59, in an effort to reduce the amount pressure loss from liquid (mineral oil) compression.
But, these small inside diameter hoses are also really great at trapping air. So, even if you remove the caliper and hang it so the hose is vertical, air bubbles can still stick inside the hose. And depending on the caliper design, how it hangs can also potentially trap some air. And so we have all the tricks people use, from pushing fluid not just from the top but from the bottom, flicking the lever, tapping the hose, etc.
Exactly. Same here since the 60's and up to the late 80's building rally cars.
Bikes use hose diameter too small and the bubbles adhere. Mineral Oil is a poor lubricant.
I'm thinking using a vacuum pump - same as I did w/ rally builds - in fact all my vehicles.
Mineral oil will degas. Still as much vertical run as possible is a good idea.
Tapping on the caliper also helps release trapped bubbles. They're only adhering by a few (surface tension) molecules.
If the calipers are still mushy, remove and shake/ tap; rotate position around and air will come out - under vacuum at the top end.
I have a new (metal type) - very 'old stock' hand pump, tried it on some bitchy Tektro's, I used vacuum and left overnight. It worked fabulously.
The hand pumps (made in the 70's and 80's) are a bit rare, but worth the find.
Juggling air saturated Mineral oil back-and-forth won't work that well. Turn empty space into a vacuum, nature abhors that state and fills it with whatever gasses are in whatever liquids in the mix.
Run vacuum up a bit higher, gasses boil off.
Not sure about over exuberant brake bleeding fetish either.
As pads wear lever input changes. Nothing to do with fluid.
Change pads, pedal input goes back where it was.
All the Magura brakes I obtained were sealed and no bleeding required.
I can see if I'd have to route through frame to rear, disconnecting the line.
Cutting the front line shorter doesn't fill the caliper with air.
Going to the rear, cut and install a tight silicone (plug) seal in the lever-end tube and feed through from bottom up to lever. Then cut (at least) that sealed inch off; install fittings and attach hose to lever.
Now top off and bleed. Nothing to do with caliper unless clutzed.
Changing pads why bleed brakes?
Fn'F