Which part of the brake system on my car needs to be addressed?

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Hello, I have a Saab 9-5 2011 that has a problem that my dealer seems to be unable to address. From time to time, when I depress the brake pedal and then release it, the pedal does not retract to its original position. This does not happen all the time, but when it does, I have to pull with my foot on the brake pedal in order to release the brakes. The dealer has changed the master cylinder and the brake fluid, but the problem continues to persist. I noticed that this problem is more likely to occur if I go backwards. I also noticed that if I park my car, turn off the engine, and pump the brakes, the problem disappears for a couple of days. Does anyone have any idea what else can my mechanic look at in order to address this issue? This is quite frustratingThank you for any helpful advice that anyone can offer. How long have you had this problem? A couple of months.