Clutch pedal drops, but pumps back up.

Hello everybody, I'm having clutch problems, as it says in the title, the pedal drops, and can be pumped back up if I pull over, but it tends to happen at lights, junctions, roundabouts and all the other annoying and awkward places, so I generally have to leave it in whatever gear it is stuck in and travel to my destination whilst beeping, shouting and flashing my lights, anything to avoid braking. I'm fine with it, but my wife gets all scared and wants to get out, but that would involve me stopping and that is out of the question. I recently changed the gearbox, and the clutch was new, I've bled it and no bubbles come through, just clear fluid, and the level doesn't drop. So I'm thinking that the only thing I touched when swapping the clutch was the slave cylinder, am I right to think that? Should I start with the slave or the master cylinder?, and are symptoms the same for both? Thanks.

series 3 88" 2.25 Diesel

slave cylinder gone.

sounds like master cylinder easy job. ps what model!

rubber flexi pipe expanding when under pressure?
are you losing fluid?

Interesting driving style!

Yeah Id go with the slave cylinder as it is the cheapest of the two to replace!

Had this before- can happen after changing clutch. Almost certainly master cylinder IMHO. If it were the slave, you'd be losing fluid.
Sudden slight increase in effort needed to operate the clutch due to new clutch spring diaphragm results in fluid getting past the master cylinder seals.
Replace.