I love Physics.
Just get a length of regular clear brake line. Available at almost any bike shop.So I recently went through this with my new(ish) 2021 Superveloce. Brake was completely fine for a few months, thought I had dodged a bullet, then it got all mushy. No amount of "regular" bleeding seemed to help. I'd maybe get a tiny bit of pressure that would then disappear right away.
Reading through here I noticed three things people seem to say help:
1) Remove and invert rear caliper, then bleed
2) Bleed from the ABS pump
3) Relocate the fluid reservoir to above the ABS pump.
I decided to try all three, since I was getting ready for a 600 mile trip and didn't have time to try each one alone.
I relocated the reservoir, then bled from the ABS, then bled the inverted caliper.
So Far, the brake is hard as a rock and works about as well as a rear brake on a sportbike could work.
It does make my inner engineer tingle knowing that gravity is helping the reservoir apply positive pressure on the whole system while it sits there.....