Uneven brake force on front wheels non S1

[Sune]

Went to the Danish MOT the other day, only to find my car being dismissed due to uneven brake force on the front wheels. When driving and braking without holding on to the steering wheel, it goes absolutely straight, so I couldn't quite understand it.
So yesterday I replaced the brake pads (MMC) - the old ones were about 1/3 left and looked old, but actually OK (car has done 33.000). The discs also look fine with no marks or discoloration. Is there anything else I should do? I haven't worn the new pads in yet, but I will do so before I try again Friday.
Any thoughts are appriciated...

[Gad-Westy]

Maybe worth taking the calipers apart and giving them a good clean up and possibly bleeding the system, other than that I can't see what the problem could be.

[Sune]

Looks like a typo in the heading - it is an S1 (1998)...
I'll take a look at the calibers. It is woth mentioning that I was more or less able to push back the pistons with my fingers when I changed the pads, and I work with computers all day, so it's not because I have strong hands

[JTBUSH]

Ensure there is no leak on the piston. Also take the pads out, get someone else to operate the brake pedal and ensure the piston moves out and pushes back in well.

Other than that, cant think what it could be.