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  #1  
Old 07-13-2018, 10:59 PM
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Brake Tail Lights Always On!

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!
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Old 07-13-2018, 11:52 PM
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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
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The Diseasel Thread - Everything You Didn't Know You Wanted To Know
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  #3  
Old 07-14-2018, 12:24 AM
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Quote:
Originally Posted by sun tortise View Post
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.
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Old 07-14-2018, 12:28 AM
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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.
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Old 07-14-2018, 12:41 AM
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I was shouting at the brakes

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.
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  #6  
Old 07-14-2018, 12:50 AM
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Post Brake Lights ON

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 ? .
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Old 07-14-2018, 09:17 AM
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Quote:
Originally Posted by sun tortise View Post
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.
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The Diseasel Thread - Everything You Didn't Know You Wanted To Know
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