Brake Booster Replacement Problem

[nicolas061291]

Hello dear Boxster lovers,

To start from the beginning:
I've bought my first Boxster about a month ago. It is a 1999 Model, 130k miles.
After driving some days, the break pedal felt somehow strange.
After another few days, the pedal wasn't coming up properly anymore, after releasing the break pedal.
I took it to the shop where they told me, that it is a problem with the break booster and that it has to be replaced. So far so good.
After taking the old one out they gave me a cool, that the break booster was filled with water. I've heard of that before, could be related to a clogged drain.
Good. They've replaced the break booster and cleaned the drain and told me I'd be good to go after they bled the system.
On my way home the breaks locked somehow (felt like the pads were pressed more and more on the rotors each time I pressed the pedal(maybe the break pressure didn't decrease after releasing the pedal)).
Now my car is sitting in the shop since Wednesday 05/11/2016 and the shop seems to have no idea how to solve the problem. They've bled the system several times now.
They've now called "experts" that should be able to tell them what's wrong (and are still waiting for their call back) and said that it could maybe be some "soft-/hardware problem in the control system".
I am in doubt that it is related to the control system. It seems like they've messed something up. What could that be?
Does anyone have any ideas? I don't want to buy a new control unit without any serious proof.

Thanks for reading my long question.
Thanks in advance to everyone.

Best regards,

Nicolas

P.S.: I apologize for any spelling mistakes, I'm not a native speaker. I've tried my best.