Morris Minor Owners Club

I have a 1971 Moggie and have very recently and suddenly encountered a braking issue. The brakes seem to work perfectly... 80% of the time. When driving slowly down a hill or in traffic, the more you use the brake the less it works. I have checked for leaks of brake fluid in the master cylinder and pipes, no leak, the master cylinder is full of brake fluid. I have also had all the brakes adjusted, this had made the car brake a bit more rapidly but has still not fixed the issue.
I thought with a mechanical setup as complicated as a knife and fork this would be something simple to fix, but i'm at a loss.

Any ideas?

Welcome to the forum! In what way 'the less it works' ? Blasting down a steep Alpine Pass - the brakes can 'fade' as they overheat - but this shouldn't happen in normal sane motoring in the UK. And certainly not 'in traffic'. Fading gives a rock-hard pedal - and a horrible burning smell - and very little braking! Or - is your problem a 'sinking' pedal as you use the brakes? Which could be failing seals in the master cylinder.......

Press hard on the brake pedal. Does the pedal gradually sink to the floor? If not then pump the pedal, does the pedal rise?

The idea is I am driving normally and the first time I brake after first starting out the car will be fine, and then when i brake again and again the brake pedal slowly sinks to the floor after reaching the end of its travel. This can be rectified by releasing the brake and pressing it down again and the brakes work fine again, and then the problem builds up and it turns into a vicious cycle. Will give the seals in the master cylinder a look tomorrow thanks