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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.

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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.

  • I hope this makes sense. If anything is unclear, post up a comment and I'll clarify. – Pᴀᴜʟsᴛᴇʀ2 Oct 16 '16 at 0:22
  • I have two pistons on each side – amphibient Oct 16 '16 at 0:28
  • There is certainly a direct relationship between the difference in fluid level in the reservoir and the depth of the consumed friction material which can be quantified. You can't equate such changes across vehicle makes/models, but the relationship certainly exists for any specific car, and should be nearly identical for any identical car (the size of the caliper piston and reservoir are not random between vehicles of the same make/model). This assumes you are not loosing fluid to other causes. If the wrong part of this answer was not in bold, I would have up-voted. – Makyen Oct 16 '16 at 2:28
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    @Makyen - The OP is asking about the consumption of volume of brake material as compared to the volume of brake fluid and to that I can absolutely tell you, * there is no direct correlation*. This is due to every brake pad design having different volume. You can't correlate the two consistently. You can correlate the depth of the pad as long as you use the surface area of the caliper pistons. Don't confuse the two as it would be wrong. Between two exact vehicles, it would be the same. Beyond it, there is no correlation. Note the OP remains generic concerning make/model/brand. – Pᴀᴜʟsᴛᴇʀ2 Oct 16 '16 at 2:41
  • @Makyen - I did update my answer slightly for clarification of what I am trying to say. Please, if you have further issue with what I've written, I'm happy to try and explain. – Pᴀᴜʟsᴛᴇʀ2 Oct 16 '16 at 2:42
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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

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    Also why you shouldn't top up your brake fluid between brake jobs. It'll overflow next time you put new pads on. :) – tlhIngan Oct 15 '16 at 23:01
  • Or, you pay attention to the level while doing the job and remove the excess with a syringe. – Martin Argerami Oct 16 '16 at 9:51
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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.

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