My name is ***** ***** I am a licenced diagnostic technician with over 20 years experience.
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there are 2 areas that could cause this since you have replaced the calipers a few times already.
1. the front brake hoses. As these hoses age, they get a little more constrictive and will stop the caliper from releasing all the way causing them to drag. This is usually more detectable when the brakes and fluid are hot. IF the brakes are dragging (when hot) I release the bleeder. IF a larger amount of fluid comes out and the drag is instantly eliminated, than its the flex hoses.
I have done lots on these.
2. The brake booster.
To determine if its this, you will have to get the system hot (like the previous test) and check for drag. Pay attention to how mutch drag. Release the 2 brake master cylinder bolts (this will cause the master to release away from the booster), if the drag is gone or less than the booster will need to be replaced.
I am leaning more towards the front brake hoses though, as this has been more common.
I hope i have steered you in the right direction of finding your issue.
IF you have any more questions, let me know!
I am always available for follow up questions and i appreciate positive ratings!