Need help with physics constrain

Its very hard to explain my problem, I tried to draw simple picture of my problem, and now I will try to explain it.
1 - starting position B actor is on position (0,0), it has Physics constrain with radius 500. P actor is on position(-500,0), and R(Physics constrain) is on position (0,0).
2 - B actor moving with -400 in X direction but the Physics constrain is still at (0,0) and P actor is still on(-500,0), everything work fine for now.
3 - B actor moving with -300 in X direction again and now it move the R with -200 in X direction which force P actor to move with -200 in X direction.
Here is my problem, I want the R(Physics constrain) to be fixed on B actor and allow him to move around P actor and move R(Physics constrain) only when leave P actor’s 500 radius.