Priority Group and Persistence Profile

Hi,

I have four servers for my web application. Two servers are in primary site with group priority 10 and two servers are in DR site with group priority of 5. When both servers goes down in primary the user request go to DR site and it is working fine. But when primary servers restore, the old user session request still go towards DR due to persistence profile. Please help me how to avoid this condition as all request should go to primary site if servers are available from primary site.