The brake pedal should have a spring on it to bring it back up, even if the master cylinder does not follow it. So this suggests that the problem is with the pedal or the linkage across the car.
You need to spend some time under the dash, upside down with the blood rushing to your head, and you'll feel dizzy when you get out. Lovely job.
You need to check the pedal pivot, and the linkage across the car, as the servo and master cylinder are on the left. Make sure nothing is in the passenger footwell causing problems. (Some time ago someone posted about a similar problem, which was caused by their passenger sticking their feet against the linkage. Only had the fault when carrying that passenger.)
Another issue could be the brake light switch. This has caused problems on other models too, particularly Stilo I think. When fitting a new switch, there is a procedure to 'set' it correctly. If disturbed, or occasionally it just seems to reset itself, it can be incorrect, and will hold the pedal down a little. Driving without the switch might help a diagnosis, but brings a high risk of being hit from behind, so not a good idea unless you can find a very quiet road.
When the problem occurs, ideally you need to stop and grovel under the dash to see why, checking switch, and linkage, etc. Might need a test route with thought to allow safe stopping points.
Do a search for brake light switch. Somewhere there is a nice procedure for resetting an old one. (Might be in another model thread, perhaps Stilo.) Fiat say if disturbed, must be replaced with new, as they come ready for an initial set. But occasionally new ones have 'let go' so need resetting anyway.