hi folks, i'm debugging the BRAKE light being stuck on. the subject line is wrong - problem isn't with the level switch as i wrongly wrote, it's the pressure switch.

anyway i didn't notice the brakes not working. the circuit is closed at the pressure differential switch on the single-chamber MC - the one with a single spade conductor. never having pulled one before, i was surprised that no fluid came out behind the switch (19mm); the reservoir is actually overfull (stupid shop). i haven't tested the quantity of vacuum, but door locks and shutoff work fine.

i have a two-chamber Lucas MC that i bought at the yard, and it doesn't seem to have the same switch unit, nor do i see the switch for sale separately. do i really have a brake problem? why does the MC need both a switch on a float and a screw-in switch like this?

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'77 240D, 504H, OM617.952, etc.

Is your car a 1977 240D?
Check your wiring diagram. Often several switches can activate the brake warning lamp, usually either the reservoir level or the parking brake pedal switch. If either shorts to ground, the lamp lights. My 1984 & 1985 have a separate "worn pads" dash warning lamp. Each switch has 2 wires, one going to ground (usually brown).

The reservoir level switch should be open when installed (w/ fluid) and closed if you pull the sensor out. In my 1996 Voyager, the warning lamp stayed on after a brake fluid refresh. Turned out the level sensor float was just stuck. After I rapped the reservoir, the float floated up and no more light. Good, since I was worried it was the "pressure imbalance" switch, which is down below on the combo valve/ABS block. Actually, our M-B cars should have that sensor too since I think that was a fed mandate ~1968. At least Mopar cars started it then. It may also light that lamp. Not sure where that block is on my M-B cars.

thanks bill, you are right. i checked the foldout schematic again and this is a "pressure differential" switch. is this a safety issue? how does one address the issue of a "pressure differential"? editing the subject to clarify

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'77 240D, 504H, OM617.952, etc.