I agree with Fast Eddie, You should not just push the pistons back into the calipers when changing brake pads, especially on vehicles with ABS. That just pushes the old contaminated fluid back up into the ABS servos and if you mess them up you are in for some expensive repairs.
I always recommend that you flush the brake fluid and refill with fresh fluid every 2-3 years or when every you do a brake job. The fluid is hydroscopic and will absorb moisture from the air while it is in the reservoir. The moisture will sink to the lowest part of the braking system (oil is lighter than water) and begins to corrode in the calipers and leads to sticking pistons and poor brake wear and poor brake performance.
If you open the bleed screw when you push the pistons back in, you can ususally push them back in by hand and I can actually feel if the piston is binding in the bore, indicating there may be corrosion in the caliper and the calipers need to be rebuilt or relaced. This is such a common problem that I just buy rebuilt calipers every time I replace brake pads.
If you go to someplace like Autozone that gives 50K mile or Lifetime warranties, you can continue to change the calipers with every brake job and never have to pay for new calipers again. I just take them in and tell them they are sticking and get new ones...No questions asked. I have been doing that since way back in the mid 1980's and I have done 4-5 brake jobs on some vehicles and never had to buy rebuilt calipers again. Some rebuilt calipers are only about $20 each and go up to about $60 each, but it is worth it if you like brakes that work.
...Rich
