'51 chev brakes

im trying to bleed the brakes on my '51 but fluid just keeps shooting out of the vent on the master cylinder. what's the deal?

 

How old is the master ,has it been rebuilt? Is there something blocking the fluid lines? PAUL

 

ya, im thinkin original or close to it. i asked my grandpa today hes been workin on them since they were new ya know, he figured a rebuild was in order. thanks for the reply.