Here my experience on my V2C 3U.
2 years ago, my brake was very tight and would respond immediately
with no slack.
Last season, I noticed brake fluid dripping from a drain hole in the
bottom of the fuselage.
Also my brake action deteriorated gradually.
I had not replaced the brake fluid (like I never do on my cars either)
and the fluid had gummed and was leaking around the master brake
cylinder. I replaced the original 20mm piston Magura master cylinder
with a new 16mm piston unit (mounted standard on BMW motorcycles) .
After extensive bleeding as described above, I still could not regain
real pressure.
(I realize that the smaller master piston does require more travel.)
However, at further inspection, I noticed that the brake pad on the
Cleveland caliper would withdraw upon release of the brake handle.
In other words, the caliper does not rest tight and at each brake
action fluid volume needs to be filled to bring the brake pads tight,
before brake pressure can be build up.
I decide to completely disassemble the Cleveland assembly to check for
gumming up and for overhaul.
Turns out there is hardly anything in the Cleveland assembly. Just one
standard O-ring.
New O-rings ordered. Caliper disassembled. Piston carefully pushed out
with a little airpressure from a small 12 V tire compressor.
It turned out that the aluminum caliper and piston had been corroded
substantially.
I presume from condensation moisture from high altitude flying and
mixing with the hydraulic fluid.
I lightly sanded, with 800 sand paper, the corroded section, where the
O-ring sits. Installed the new O-ring and re-assembled.
After again extensive bleeding (upwards from the caliper to the
reservoir), I now have pressure.
I still need to test fly to see how well the overhaul works.
I am not yet sure if this has resolve all problems as I now have
substantially more "squeeze" than before.
But as mentioned, this could be partly due to the smaller master
piston, which would give more throw but also more force/pressure.
Summarizing: My first problem was the leaking master cylinder. After
that next problem was piston in the Cleveland caliper retracting upon
brake lever release (instead of taking new fluid from the reservoir).
I am still not sure if this retracting is a problem of the new master
cylinder or a problem of the corroded Cleveland brake caliper.
3U