There is no real accurate advice on this one Graham.
June 2015, I bought a 6 year old Impreza and Subaru's in my experience can get sticky calipers.
Checked the car with an IR temperature gauge and the front and rear offside calipers were 15 to 20C hotter than the nearside after driving but not braking.
I purchased Subaru seal kits and did both the offside calipers. 18 months on and the front caliper is fine but the offside rear one that I rerubbered had the piston go rusty.
This time I replaced the offside rear caliper over Christmas 2016.
When I did both calipers I probably used 2 ltrs of fluid to make sure there was no old fluid in system. On the way down to the Lake District in February this year we were stopped at some roadworks near Pulley Bridge and with nothing to do I went round the car with the IR temperature gauge again and the nearside rear caliper was just starting to seize. It was at say 45C when the replaced offside caliper was say 25 to 30C.
I changed the nearside caliper when I got home. It appears to me that there is a local problem with metal salts from farmers fields being leached on to the roads near where we live and that appears to get through the caliper dust seals.
I have always found rear drum brake cylinders in UK supplied cars a leak just waiting to happen.
