Air in your system regards your foot brake, at the rear of you car the brake cable run through outer sleeves that cannot be lubricated, you can check the handbrake route by jacking up the car ( SAFETY IN MIND) & check the cable route for road muck, if you can raise the car clean if possible the route of the handbrake & direct WD40, the way the weather has been of late salt & grit has entered the mechanism at the (IF DISKS) disk, if a drum the Mechanium to a point is protected but grit can still enter the drum.