We had a weird one years ago on a MK2 16V. The brakes would be awesome one day and dire the next, we couldn't put it down to a particular set of circumstances.
It took a while but we eventually came to a loose front wheel bearing. The pads were getting knocked further back by the angle the disc was getting to. So you were effectively straightening the wheel up before doing any braking.
Sometimes the seals fail in the master cylinder and the fluid can get past in either direction. You don't lose and fluid but the brakes seem ok on the first push and then sink the longer you hold them.
On my Ibiza the brakes get a bit lazy driving through town all the time. If I stand on them hard a few times, while at sat at the lights the pedal goes really hard and then the brakes are much sharper for a couple of weeks.
Gavin
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