I've been wondering lately, since I've been doing a number of brake jobs, about the distance between the high and low marks on my brake fluid reservoir. As you know, as your brake pads wear, the pads move outward to account for the wear. The fluid level in the reservoir will become lower as a result. Is the level at which the switch will illuminate the "brake light" on the dashboard at such a point that there is still brake pad left? Is it a warning to change your brake pads?
Several times during a brake job I'll push the pucks back into their bore and the reservoir will overflow. That should only happen because someone added brake fluid to "top it off". Doing so would render the "check brake light" useless.
Are there any automotive engineers or bake experts that would care to comment?

 

No engineer, have done plenty of brake jobs. I never push fluid back in a brake system, open the bleeder and push the dirty old fluid out.

 

That's the perfect time to do a brake fluid exchange during a brake job.. Like Eman said, drain it out.

 

Working on European cars for well over 40+ years, it's common to do a brake fluid flush every two years. When I started, it was a yearly maintenance item, but then oil services were done every 3,750 miles.

When I started in this profession, I thought it was pretty stupid but as I got older, I noticed, we never had brake hydraulic issues. I wonder why? Everyone should do this, domestic or not.

With new brakes, the fluid will be full. I have never seen a brake reservoir so low that the light came on, even with all 4 brakes worn out. I tend to look at the reservoir to determine what the brake pads are doing. You should never add fluid to the brake system. If you have to because you have a leak, you need to fix it. If you ever see it low, check the brake pads.

As for pushing 'old' brake fluid back into the system, I do every single time because the brake fluid is never 'old' in my clients cars. I charge around $75 for a brake fluid flush so it's not a money grab for those that think I ripping my clients off.

 

Many reasons not to push fluid back into a system, not as much to do with the cost of brake fluid. Pushing dirt or whatever back through the ABS controller and into the MC just doesn't make sense. Brake fluid attracts moisture which is another reason to change fluid and clean out the MC reservoir.