Brakes working on and off

A few days ago I noticed that I was loosing pressure in my break pedal while I was driving, so I turned off the car, pumped the pedal until I had pressure(3-5 pumps) and as soon as I turned on the car the pedal lost almost all pressure and went down to the floor. So I checked the brake fluid which was fine, checked for leaks which there weren’t any, so I thought it was either a brake booster issue or a master cylinder issue, so I replace the pair of them and drive it around for a day with no issues. When I woke up in the morning to go to work, theres intermitant pressure bro my brakes now and sometimes they lock up too. Is it a vacuume line that’s cracked and loosing pressure? What’s the deal? Any thoughts?