Zip tying the lever won’t do anything except maybe prove the piston seal is worn.
If your lever won’t hold, buy a master cylinder rebuild kit ($20).
Remove the lever
Gently remove that piston dust cover–i used needle nose but try not to tear it. The new kit comes with a new cover.
Use snap ring pliers to remove the snap ring from the housing–you’ll probably curse blue smoke trying this. Then remove the piston.
The piston seal goes in one way–take careful note Of the position.