agreed. go lower. below 500 microns is better but lower is best. lower and holding is an even better proof to a non leaking system. keep in mind that some things leak under a vacuum and some things leak under pressure so continue to leak check. since this is a unit that has had a lot of issues, you need to keep going until you checked everything several times and then several times again.
This is solid advice considering you have no idea how long the system was open for. Sometimes bigger machines take forever to get to 500 microns (days not hours). With that being said, if this isn't a critical unit, take your time and do everything correctly the way it needs to be done.