Mastiff brake bleeding

[Mimmi123]

Hi everybody!
I am new member and I have a Mastiff with brake problems. I took the bike apart to give it an overhaul.
I had to replace the front master brake cylinder because it was leaking like a sieve.
I used an old, but working, Yamaha YZF 125 master cylinder I had in a box of left overs.
Oddly enough, I can´t seem to be able to bleed the front caliper, no matter how I go about it.
I have had it off the bike, but not taken it apart.

The same problem with the rear brake. Can´t bleed it. I have tried the normal way as well as using vacuum to suck the fluid through the bleeding screw.
Nothing happens.
I need all the knowledge of this forum.

[DAVID THOMPSON]

some have had a problem with the piston in the master cylinder not returning to its proper
resting point so the hole does not open to allow new fluid into the mastercylinder

if the piston does not return to a proper resting place no new fluid can get from the storage tank
and into the system

this can happen on both the front and rear from a poorly adjusted break lever not allowing a full piston return

when working on my rotax powered saxon tour i had to push fluid up from the brake to the top
the air just refused to flow out of the system in the down direction

also plastic speedo faces and windshelds HATE BRAKE FLUID paint does also