This is a common scenario. You replace your worn out brake pads with new pads and lo and behold, you can't get the pistons in the brake caliper to retract fully, so with your new pads installed, there's no longer room for the rotor.
There are a few common causes, and the answer typically depends on what type of brake fluid you are using.
DOT fluid brakes
If your brakes uses DOT 3, 4 or 5.1 fluid, then it will absorb water over time. This will result in an increase in the volume of your brake fluid. You won't notice this as your pads wear down because the system can manage the increase in volume. However, with a new pad change, the symptons of pistons not fully retracting will rear their ugly head.
The quick and dirty solution is to crack open the bleed port on the master cylinder (at the lever) to let some of the excess brake fluid out as you push the pistons back in.
The better approach is to replace the brake fluid with fresh stuff and then give them a complete bleed.
It doesn't take much absorbed water in DOT fluid to dramatically lower its boiling point. If you boil your fluid, then the bubbles that form inside the lines can leave you with no braking when need it most.
Mineral fluid brakes
Mineral fluid doesn't absorb water, so its unlikely that the fluid has swollen to cause your pad retraction problem. Check out the common causes for what might be causing the problem.
Common causes (all brakes)
Here are a few things it might be:
- aftermarket pads: sometimes, aftermarket pads are thicker (or their manufacturing tolerances are lower) than OEM pads;
- air in the system: if you can push the pistons right back, but they come back out, then you might have air in the system. Put your bleed block (or anything suitable) between the pistons and gauge the lever feel. Is it spongy? If so, give your brakes a bleed;
- "sticky" piston or pistons: symptoms are similar to air in the system (but without the spongy lever feel). The brake piston seals are designed so they deform slightly when the brakes are applied. When you let off the brakes, they untwist and help to retract the pistons for drag-free running. If the piston doesn't slide nicely on the seals, then the seals can stop the pistons from permanently retracting when replacing pads. The fix is to carefully pump the brake lever so both pistons come out a little from the caliper. If only one piston is coming out, then put pressure on that piston to hold it in place and see if you can get the other coming out equally. Give the piston sides a good clean and then apply a very light coating of brake (or rubber) grease. Push the pistons back into the caliper.
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