[SmellyGeekBoy]

Really? Years back I had the brakes completely fail on a classic Mini I'd worked on because one of the pipes wasn't fully tightened and all of the brake fluid pissed out the first time I applied the brakes. Thankfully I managed to grab the handbrake use that to stop (the Mini handbrake is cable powered and independent of the hydraulic system).

Do modern cars have anything to mitigate this?

[UncleEntity]

Pre-modern cars have something to mitigate this, testing your brakes more than a single pedal push before you go out on the road. Kind of sounds like you didn't even bother to bleed the lines which is also asking for a heap of trouble even if you managed to properly repair the entire braking system.

I've worked on the brakes on multiple cars and never once took one out for a test drive without triple-checking that I didn't make some stupid mistake that would put other people in danger.

Don't get me wrong, I had a couple cars with dodgy brakes over the years but always made sure they were good enough at the time I was driving them -- had this '53 Dodge that I had to bleed the brakes every time I wanted to take it out for a spin because the wheel cylinders were pitted and slowly leaked but always made sure to do a proper job because other people's lives were at risk too.

[chiph]

My understanding is that not all classic Minis had dual circuit hydraulic brakes. So if you have one of the originals with a single brake circuits, any leak means you will fail to stop (if fitted with drum brakes, stopping was doubtful anyway)