Brake Tail Lights Always On!

[sun tortise]

Dealing with my endless lack of brakes in a 300CD, i changed the master cylinder and have some improvement, but just noticed that THE BRAKE TAIL LIGHTS ARE ON WHENEVER THE IGNITION SWITCH IS IN THE RUN POSITION, WHETHER THE BRAKES ARE STEPPED ON OR NOT! I jacked up each wheel to see if it spun freely, both with and without the engine running, and all four spin freely. Tried pulling the electrical connectors off the reservoir, made no difference. Am baffled, and wondering if there is some connection with the brakes working so poorly despite nearly every part in the system having been replaced, and repeated attempts at bleeding the brakes. Where is the switch that controls those lites? Thanks!

[Diseasel300]

You're not the first to struggle with bleeding brakes on these cars. My experience has been that 2 people are required (one operating the pedal, the other cracking/sealing the bleeder port) or a pressure bleeder is required for good results. A one-man show simply won't cut it.

The brake light switch should be above the brake pedal. I really haven't looked at the Mercedes switch, but a friend of mine had his brake lights stick on due to a rubber bumper falling off the pedal arm of his Hyundai.

Quickie DIY page for a changeout from the forum sponsor:
https://www.pelicanparts.com/techarticles/Mercedes-W123/63-ELEC-Brake_Light_Switch_Replacement/63-ELEC-Brake_Light_Switch_Replacement.htm

[Frank Reiner]

Originally Posted by sun tortise

Dealing with my endless lack of brakes in a 300CD, i changed the master cylinder and have some improvement, but just noticed that THE BRAKE TAIL LIGHTS ARE ON WHENEVER THE IGNITION SWITCH IS IN THE RUN POSITION, WHETHER THE BRAKES ARE STEPPED ON OR NOT! I jacked up each wheel to see if it spun freely, both with and without the engine running, and all four spin freely. Tried pulling the electrical connectors off the reservoir, made no difference. Am baffled, and wondering if there is some connection with the brakes working so poorly despite nearly every part in the system having been replaced, and repeated attempts at bleeding the brakes. Where is the switch that controls those lites? Thanks!

1) Which lights seem to be the problem; the tail lights, the brake lights, or both?

2) In any event, both the tail lights, and the brake lights are very sensitive creatures, and will behave badly when shouted at.

[BillGrissom]

I think it is just a switch on the pedal, like all my other cars, though I don't recall ever looking for it on my W123 cars. There is generally an adjustment. It should sense the pedal motion without affecting it, i.e. pedal shouldn't bottom-out on the switch. Good it is only powered in "run". In many cars, it is always powered, meaning the battery runs down overnight if the brake lights stay on.

[sun tortise]

and cussing even. Thanks for the link to how to get at that switch. Very clear pictures. I was bleeding the brakes with the tube in a jar method, which has worked for me in the past. maybe i will try to make a pressure bleeder out of a spray pump as i saw in another post.

[vwnate1]

The take away here is very simple :

Unplug the brake light switch, if they go out, either the switch is out of adjustment or it's bad, easy fix either way .

Now, back to bleeding ~ I use a pressure bleeder, co$t me $50 for a Motion Pro brand one, maybe your F.L.A.P.S. has one as a loaned tool ? .

[Diseasel300]

Originally Posted by sun tortise

I was bleeding the brakes with the tube in a jar method, which has worked for me in the past.

That's worked for every other car I've ever touched. Has failed miserably on both Mercedes, and I do mean miserably. Left the SL with brakes that wouldn't stop the car and the SDL with a a spongecake for a brake pedal. A 2-man setup worked for the SDL, but the SL required a pressure bleeder. Not sure why Mercedes brakes are so hard to bleed, but the tube-in-a-jar method does NOT work.

[sun tortise]

Finally got someone to help me bleed the brakes. Got a good fluid flow on all of them, but the pedal still goes way low before the brakes engage. Wheel calipers and pads were replaced by P.O.



Wondering if there is some kind of an adjustable arm that affects how much the pedal needs to be depressed to activate the brakes.

[TurboRedkneck]

Is there a pedal freeplay adjustment on these cars? Maybe someone else who has been in there will chime in. I haven’t replaced booster or master on my benz, but on my truck there was an adjustment in the rod going through from the pedal to the booster. When you fixed the lights, did you move the switch closer to the pedal? If so, maybe the pedal wasn’t coming all the way up. Also, I presume you did a bench bleed an the master before install?

[vwnate1]

It requires -both- a pressure bleeder and the bottle with rubber hose connected .

Yes, those of use who do brake bleeding on a regular basis can usually do it single handedly without the jar by feel but this still sprays corrosive brake fluid underneath the vehicle's chassis and is simply lazy and bad practice .


The order in which wheel is done is also very important and not mentioned by the O.P. .

For satisfactory results one must dot every 'I ' and cross every 'T', every time .

[sun tortise]

Got enough brakes to take it for a spin, and the pedal goes almost all the way to the floor, and the orange brake warning lite comes on. Really baffled, as almost every thing brake related was replaced, bled, etc.

[vwnate1]

? Can you post a picture of the master cylinder and resivoir ? .

[sun tortise]

Took the pix, but they won't upload, not sure what i am doing wrong. Looks like a normal master cylinder to me. Both sections of the reservoir are almost full. Same problem with the one that was on the car, and one that came with the car that the P.O. said was new.


Not sure what to look for or how to make the picture attach.

[vwnate1]

This site often has problems with photos, there's always a dialog box telling you why, too large, already been posted before, etc.

Send 'em to me and I'll give it a go as I had a similar problem with my gray market 300TD .

VWNate (1) (at) yahoo (dot) com

[sun tortise]

After many tries, and lots of help and suggestions from all of you, especially VWNate, i got those brakes to work pretty well, but still required pedal to go nearly to the floor, and the orange brake warning lite would come on. Well i sold the car, and the folks who bought it said they got a few miles down the road, and the brakes SEIZED and the rotors became red hot! Not sure if it was front, back or both. I sent them a hot brake rebate.



Not my headache any more, but i am baffled as hell about it. Could it have been a "proportioning valve"? Wrong brake booster?


Thanks.