Peugeot 206 1.4L automatic Brake Problem

Can anyone help me with this please? I drove into the back of someone a few weeks ago but it wasnt a bad accident or anything and at the time I had to slam on the brakes. Since then my brake pedal has been very loose. I have had the peugeot dealer check it out and he says everything is fine but I know that its not. Before the crash when I would apply the brake there was always a good amount of resistance coming back from the brake and I was used to it. But now the brake pedal is very loose and I have to push it all the way in practically to get the car to stop. Can anyone help me with this? I dont feel comfortable driving the car while I have this problem.

When the car went in to the crash repair centre I told them about the brake and they sent it to their local peugeot dealer to have a look at but they came back and said that there is nothing wrong with them. But to stop the car I have to put my foot all the way in on the brake pedal.

I have been onto the garage again and told them its not right. Im waiting for them to get back to me today. Before the brake pedal was very stiff and I only had to push it in no even halfway to stop the car but now its all over the place. Very annoying. Ive been reading up on the net. Could it be anything to do with this master cylinder? Or this vacumn thing maybe? I am bringing it to a brake test centre next tuesday. Is there anything in particular I should ask them to look at?