Got my new lude through an MOT recently no sweat but I did notice while I was working on it that one of the braded hoses was weaping, no problem I thought so I swapped them to stock one from another breaker I did a few years ago. I bled them good, no air left coming out and topped up the res using dot 4 fluid as I was going along, it did not run dry at any point. All seemed fine for a bit but then my brake pedal randomly goes down a lot further that normal, brake again and it is fine
I bled them again for good measure and same thing has happened to me again

Its all dry and no obvious leaks, bled twice. Its got me baffled, am I using wrong brake fluid?? is my master cylinder on its way out?? Was not doing this before I swapped the flexi pipes over. Real pissed of because I didn't expect this and I know I will end up working in the drokking rain.
Any ideas please chaps? Cheers.