Omega B – Twitches while braking

Hello guys. Here’s the omega B from 2001 with the 2.2 mashie and automatic transmission. The following problem when braking: From about 40 km/h when you step on the brake pedal from medium force he pulls the track short. Feels he pulls about right, left and then he stays straight. If you let go of the brake the same reverse. Had the brake discs and pads renewed because these were already shortly before wear. Seeing that a piston in the saddle(HL ) did not go back completely. Someone else an idea? All parts are from well-known manufacturers and no cheap scrap. Or even someone had the same problem.? Unfortunately I have only 1 small jack to check at the moment. Otherwise I have to drive to the workshop.