The brake fun just keeps on comin!

A few weeks back I found brake fluid in the engine bay below the master cylinder and the rear system's brake fluid reservoir was bone dry. I noticed the bolt that screws the rear-wheels' brake line into the bottom of the master was wet and could be tightened a good 3/4 turn, so I tightened it, bled the master, and the pedal felt great. Noticing both wheel cylinders were leaking, I replaced those this past week.

I tried to take the car out tonight to test the brakes and noticed I needed to pump the pedal up to get the car to stop when coming out of the driveway. I checked in the engine bay and a fair bit of brake fluid was sneaking out between the brake line's bolt and the bottom of the master again despite tightening that bolt a few weeks prior. Is there a plastic/rubber seal there that has failed indicating I need a new master brake cylinder, or is that metal on metal which is now leaking for some reason?

The car has been sitting for the better part of 9 months while I redid the floor's sound deadening/carpeting and it's odd that the entire rear brake system seemed to die at the same time.

The wheel cylinders were last changed in 2016 and the master was changed in 2019.

Edited 9 hours ago9 hr by chaseincats

If the paint on the booster below the mounts of the MC has been eaten away sounds like the master is bad. Start there. Seating the lines on the master cylinder can be tricky.

The nut on the brake line into the master cylinder is a metal on metal seal. If you have factory lines or CuNi lines then you should be able to get it to seal. If they're stainless, they can be much harder to get sealed