Hi Alex,
I fully dismantled the caliper: removed it from the car, removed all seal and also took out the piston. I found light rust on the top of the piston as there was heavy rust where the metal clip holds the dust seal into. I cleaned this all and reassembled. I bleed the caliper with an Visiibleed and had no bubbles. (I will check again on the weekend with my compressor draw through tool)
I recently bought this to see if i have any air leaks when pressuring the system as i am not loosing any fluid but the cap is different.
I noticed the inside of the piston (the area facing outwards) is very rusty and flaky so i too removed any loose pieces and ran a dremel sanding attachment inside.
On assemble i ran the piston in and out.
I'm wondering now if the rust inside the piston is making it imbalanced / if i need a new seal inside the caliper / if a used/new caliper will fix the problem.
Do these cars have valves for the rear brakes?
I'm also going to check the flex pipe to make sure its not expanding this evening.