The red BRAKE light is staying on in my 77 300D. I recently replaced the master cylinder and cleaned the reservoir using simple green and rinsed with water. The reservoir is full.

The early W123s have one sensor on the master cylinder and one sensor on top of the reservoir. Any ideas where the problem may lie? Could the sensor on top of the reservoir be damaged?

Should be easy enough to check, unplug the sensor and see if that cures the light. If it does, then either the sensor is bad or the reservoir isn't really full.

The reservoir has two chamber, front and rear, and the rear chamber can be REALLY hard to see the level. Try adding more fluid, slowly so as to avoid spilling.

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/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

Thanks. I'll give that a try. I had another question. What is the proper level of brake fluid for these cars? The MAX mark has a carrot on top of it. Should the level be below the MAX text or below that carrot?

On another note, any idea if the sensor inside the reservoir is replaceable? I can't seem to find them anywhere.

I think those float switches just close when the reservoir is at the appropriate level.

You can remove the connector and jumper across their pins with a wire. If the light goes off on the dash then you've got a bad float switch in the reservoir.

If not..then I think the only other thing that can trigger that light is the parking brake. Check and make sure yours is released and not contacting it's switch.

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In my 84 & 85, the rubber caps on the 2 reservoir level sensors must not be pressed down too much or the float cannot come up enough to trip the switch. Those caps are called "test caps". You push down and insure the dash light lights. I wouldn't though since the rubber is usually brittle and you don't want cracks that will allow moisture to the glycol fluid.

Just found out the problem. The sensor on the master cylinder is causing the light.

I wonder why... It's a brand new MC and a brand new sensor. Any idea what the purpose of this sensor is?

That's a brake pressure warning switch. I "think" it is there to detect a difference in pressure between the front and rear portions of the brake system. Another way of warning you that you have a leak or other problem in either the front or rear brakes.

Are you still having trouble bleeding them? I didn't see if you updated your thread on benzworld about bleeding the brakes.

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Got them bled yesterday. The wife and I did about three cycles, then a big burst of air came out. We kept doing it until I made sure there were no issues.

Drove the car and it works fine.

I've triple checked all of the lines and see no leaks. All of the soft lines were replaced.

On another note. I pushed the reservoir down as far as it will go, however, there is still a little gap between the bottom of the reservoir and the top of the MC gaskets. There are no leaks, however. I wonder if this might be some of the cause of the brake light...