Here is a guide that helps with this issue. If the master cylinder is still full then I suspect it has an internal leak that is allowing the pedal to go to the floor.
Review this guide and let's go from there:
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor
What I would suggest is capping the lines coming off the master cylinder and then push the pedal. If it still goes to the floor then it has to be the master cylinder as you eliminated everything else in the system. If it is a hard pedal immediately then you have a leak somewhere else in the system.
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Friday, April 2nd, 2021 AT 9:58 AM