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Is it normal that the level of brake fluid in the tank rises after you replace brake pads, approximately for the volume of how worn the old pads were?
E.g. if your old pads were down 1/4" in thickness relative to new ones, and the surface of each pad is, approximately 3 sq in, the volume of fluid in the tank should be higher by 4 pads x 3 sq in surface x 1/4" recovered thickness = 3 cu in.
While there is going to be a correlation between the loss of friction material and the amount of fluid in the reservoir, there is not a direct relationship you can quantify. Every make/model/brand of vehicle is going to be a bit different as far as the size of the caliper piston and the size of the brake pad supporting the stopping power.
The amount of fluid is going to equate to the amount of friction material which is gone, multiplied by the surface area of the piston which is in the caliper. The size of the piston is what's going to relate to the amount of fluid missing from your master cylinder, not the amount of pad which has been consumed. The surface area of the pad does not come into play (the depth of pad used does, because the piston will travel this same basic amount).
Another factor to consider when looking at this is, most calipers used on passenger cars today have a single piston which is located on one side of the caliper. What happens when you have multiple pistons on one side of the caliper, or multiple pistons on both sides of the caliper. You could have pads with the exact same surface area, but could be utilizing far more brake fluid as the pistons move further out of their caliper bores.
Yes, as brake pads wear down and lose thickness the piston in the brake caliper has to compress more to make up for the lost area. The piston requires brake fluid to compress so the more it is compressed the lower the fluid in the reservoir shows because it went to the brake lines. That's why a low brake fluid might be a sign of worn pads
Brake fluid will move back up into the master cylinder reservoir when the caliper pistons are pushed back into the piston bores to ready the calipers for the new, thicker brake pads. This, however is not best practice. Best practice is to open the brake bleeder ports while the pistons are pushed back into the bores. This is done so that dirty fluid that could contain contaminate solids not be reintroduced into the master cylinder bores where they could chip the cylinder seals or score the bore. The volume of fluid would be equal to the volume in spaces behind the caliper piston bores. This then is would refill the reservoir and could cause it overflow.
Brake fluid best practices include the aforementioned no back-flush rule. Flush entire system at no more than three year intervals. Keep the reservoir full at all times to allow extra volume in case of a leak. Keep the cap on at all times to keep the fluid from absorbing water from the atmosphere.
