Welcome to 2CarPros.
Chances are the switch is going bad and sticking (or the fluid is low). The switch is in the cap and basically functions as a float. As the fluid goes down, the plunger drops. At a specified point, it actuates the light.
You can try removing the cap and moving the plunger up and down to see if that takes care of it. Other than that, it needs replaced.
By the way, this cap doesn't come with a new master cylinder.
I attached two pictures. Picture 1 shows that the cap is the switch and picture 2 is an exploded view of the cap and reservoir.
Let me know if you have other questions or need anything.
Take care,
Joe
Images (Click to make bigger)
Wednesday, September 30th, 2020 AT 5:54 PM
(Merged)