After some use the master cylinder piston can get sticky. A clean and re-grease Will have it performing like new again.
Remove the circular caps from both sides of the brake lever pivot.
Press the pivot pin out of the lever. Exercise caution to only press it out far enough that you can remove the brake lever.
Use a suitable pin punch to press out the master cylinder retaining pin. It is the silver pin in the picture below. Like the lever pin, only press it out enough to clear the master cylinder bore.
Give everything a good clean.
Extract the master cylinder plunger by pulling it out gently.
Clean the piston seal. Take care not to damage it. Regrease and reinstall.
Reverse the steps above. Once done, give the system a complete bleed.