Clutch pedal goes to floor and stays there? Don't panic, pull it back. I fitted some rope to pedal.  Dealer can't define what is wrong. Can be pedal, master cylinder or  slave cylinder (gearbox removal job.) Cost may be £1000. Mine was master cylinder, under extended warranty, priced at £500. 'Badly machined' component!

They are feeding you Bull***t as is common, there is no machining on the master cylinder, its a plastic moulding with a moulded plastic operating rod and a single plastic piston with a sintered toroidal magnet to activate the pedal sensor. It cannot be disassembled unless cut apart with a grinder or making special tooling as it is clipped together during manufacture.

 

The fault will have been the concentric slave cylinder drawing in air through the O ring joining the feed pipe to the cylinder, more moulded plastic parts, lifting the pedal up causes a partial vacuum and draws even more air in.

 

If they really did replace the master cylinder the only reason it is working now is because the system has been bled, the fault will reappear unless they actually replaced the slave cylinder, its an odds on chance that they may have done as they are story tellers.