I changed my brake fluid and am getting a low brake fluid warning even though the master cylinder is full. I wonder if I may have damaged the float in the process and is this something that requires a new master cylinder?
Thanks for any input and/or suggestions.

 

There are two pins that secure the reservoir to the master cylinder. The fluid level switch snaps into the reservoir at the lower center of the reservoir. (See attached photo. Switch connecter can be seen in the lower left of photo.) I ended up removing mine and cleaning it out. There was a fair amount of gunk that had build up in it. This might be your issue as well.



This is a picture of the switch.



This shows the reservoir just before reinstalling.



This is the cleaned, freshly installed and filled reservoir.

 

Did you find your problem??

 

The brake fluid sensor snaps into the bottom of the reservoir, but is not actually in contact with the fluid or inside of the reservoir, so it is not likely you damaged it.
However, if it does not work, check the connector, and then replace the sensor if required.

 

So how the heck does it work then??

 

There is a small float with a small magnet. If the fluid level becomes too low the magnet comes closer to the bottom of the reservoir and will trip the reed switch that snaps into the bottom area of the reservoir. The reed switch is in the black plastic piece I’m holding in my fingers in the posting above in photo.

 

Nice write-up and photos Eric D!

 

Science is fun. It sounds like the float mechanism can still jam up if crud gets into it. Thanks for the explanation!

 

Back in 2015 - I replaced the sensor, still got the warning light. If I remove sensor from the reservoir, plugged in, the warning goes away. So the problem is the sender - the magnet in the reservoir is stuck at the bottom, tripping the low brake fluid warning.

You can see that magnet with the cap and fluid removed, it rides in a vertical housing in the middle of the reservoir. With the fluid out, you might just see a 'black line' there but that's the slit where brake fluid goes in and floats that magnet away (up) from the sensor when there is adequate brake fluid. Mine was stuck in there due to really funky dirty fluid. Get a slender 6" or longer piece of metal that will reach in there and get in that vertical gap to clean it out, and then move the magnet up and down until it moves freely. A FLATTENED (hammer it) piece of coat hanger works perfectly to get in there and 'exercise' the magnet a bit till it moves freely.

I did that, refilled brake fluid and, no more warning light. Had a full professional flush done

Above was 2015, it happened again Aug 2018, though I had a pro brake fluid flush and replacement done back then. This time when I drained the brake fluid (using a turkey baster, works great!) there was crud in there again though not as bad as last time. Once again used a narrow piece of wire to work the magnet free in that slot and after moving it around a lot, and cleaning out the one (right) side of the fluid housing I could get to, I topped it off with fresh fluid and no more warning light. However, clearly I need to remove that reservoir and use some kind of cleaning fluid to get in the left side which is probably really gunked up.

 

Although this reply was a bit dated, it sure helped me out. A flattened coat hanger cleaned out the slot and fixed my low brake fluid light. Thank you!