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  •  » FYI Ford, Classic Mustang Tech Discussion
  •  » Brake light switch and intermittent brake light operation.

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    2/09/2025 4:58 PM  #1

    moon4964
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    Registered: 9/04/2024
    Posts: 46

    Brake light switch and intermittent brake light operation.

    My 1964 Falcon uses the brake light switch operated by brake fluid pressure not the mechanical pedal-mounted type. The switch is mounted on the master cylinder. Changing to a pedal-mounted switch is not a viable option right now.
    Brake light operation is intermittent, mostly inoperative.
    The car's wiring is perfect to the brake lights. For instance, I can connect the two wires (+12VDC and ground) attached to the pressure switch and the brake lights illuminate. I suspected the brake light switch, so I purchased a new Standard Brand switch. To test the switch prior to installation, I connected an ohmmeter across the two terminals, then depressed the plunger inside the fluid port. There was no continuity between the terminals. Since the switch is a fluid pressure switch, I believe there should be continuity when the plunger is depressed. Am I missing something?
    I returned the switch to O'Reilly's until I sort this out. They ordered another one for me should I choose to try again.
     

     

    2/09/2025 6:00 PM  #2

    RTM
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    From: Cape Coral FL
    Registered: 5/22/2019
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    Re: Brake light switch and intermittent brake light operation.

    moon4964 wrote:

    My 1964 Falcon uses the brake light switch operated by brake fluid pressure not the mechanical pedal-mounted type. The switch is mounted on the master cylinder. Changing to a pedal-mounted switch is not a viable option right now.
    Brake light operation is intermittent, mostly inoperative.
    The car's wiring is perfect to the brake lights. For instance, I can connect the two wires (+12VDC and ground) attached to the pressure switch and the brake lights illuminate. I suspected the brake light switch, so I purchased a new Standard Brand switch. To test the switch prior to installation, I connected an ohmmeter across the two terminals, then depressed the plunger inside the fluid port. There was no continuity between the terminals. Since the switch is a fluid pressure switch, I believe there should be continuity when the plunger is depressed. Am I missing something?
    I returned the switch to O'Reilly's until I sort this out. They ordered another one for me should I choose to try again.
     

     
    I have no experience with those switches but I agree, it should be open when no brake pedal pressure or s applied and a closed circuit when you do apply pressure.

     

    2/09/2025 6:35 PM  #3

    John Ha
    I FYI 24-7

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    From: Mitchell, SD
    Registered: 4/02/2013
    Posts: 950

    Re: Brake light switch and intermittent brake light operation.

    I have no experience with those switches either but the brake system typically has fluid pressures in the hundreds, and sometimes thousands, of pounds.  If you didn't apply that much force to the new switch plunger, it may not have been enough to close the contacts.  Personally, I'd get the replacement, install it, and test it as it was intended to be used.

    Last edited by John Ha (2/09/2025 6:36 PM)

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    2/09/2025 11:02 PM  #4

    Bullet Bob
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    From: Montrose, Colorado
    Registered: 4/01/2013
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    Re: Brake light switch and intermittent brake light operation.

    What John Ha says....plus, back in the 60's when I worked as an Auto Eeee-Lec-Trician for Sears and Sawbuck, I bet I replaced over 50 of those hydraulic switches in the few years I works there.  Replace the switch.  If bypassing the switch makes the lights work, there ain't much else that can go wrong.  Maybe some weird issue with the master cyl, butt (TS&T), I doubt it.

    "you get what you pay for, good work isn't cheap, and there are NO free lunches...PERIOD!"
     

    2/10/2025 7:32 AM  #5

    BobE
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    From: White Plains, NY
    Registered: 4/04/2013
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    Re: Brake light switch and intermittent brake light operation.

    I agree w/John Ha & BB.  Replace the switch.  Brake line pressure is a lot higher that what you can apply by pushing on the plunger. 
    Going back some time but I replaced one on my 61 Falcon and didn’t bleed the brakes afterwards.  As the switch is at the top of the system, no fluid leaked out, and the brakes worked w/o any issues.
     
    I’ll add that a friend father’s had a 64 Galaxie.  The car would set in the garage for days and the battery would die.  They were in the garage at night and noticed a slight red glow against the garage door and found the rear brake lights were dimly on.  Changed the brake light switch on the MC and the problem disappeared.

    65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.
     

    2/10/2025 9:26 AM  #6

    Bullet Bob
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    From: Montrose, Colorado
    Registered: 4/01/2013
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    Re: Brake light switch and intermittent brake light operation.

    Yup Bob, that's another thing about those danged hydraulic switches...they can and will cause a low current drain that will only barely make the filaments glow, but will run the battery down. 

    "you get what you pay for, good work isn't cheap, and there are NO free lunches...PERIOD!"
     

    2/10/2025 2:15 PM  #7

    John Ha
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    From: Mitchell, SD
    Registered: 4/02/2013
    Posts: 950

    Re: Brake light switch and intermittent brake light operation.

    BobE wrote:

    The car would set in the garage for days and the battery would die.  They were in the garage at night and noticed a slight red glow against the garage door and found the rear brake lights were dimly on.  Changed the brake light switch on the MC and the problem disappeared.

    Oddly, I had a similar experience, although with a different cause - the battery would die after a few days of sitting.  I was out in the garage one night just checking to make sure everything was closed up and noticed a glow coming from under the dash.  Turns out the glovebox light was never turning off.  That was promptly and joyfully fixed.

    Last edited by John Ha (2/10/2025 2:21 PM)

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    2/10/2025 7:09 PM  #8

    moon4964
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    Registered: 9/04/2024
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    Re: Brake light switch and intermittent brake light operation.

    I will report back in a week or less with the final answer.
     

       •Thread Starter
     

    2/11/2025 1:32 PM  #9

    6sally6
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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,884

    Re: Brake light switch and intermittent brake light operation.

    John Ha wrote:

    BobE wrote:

    The car would set in the garage for days and the battery would die.  They were in the garage at night and noticed a slight red glow against the garage door and found the rear brake lights were dimly on.  Changed the brake light switch on the MC and the problem disappeared.

    Oddly, I had a similar experience, although with a different cause - the battery would die after a few days of sitting.  I was out in the garage one night just checking to make sure everything was closed up and noticed a glow coming from under the dash.  Turns out the glovebox light was never turning off.  That was promptly and joyfully fixed.


    6s6
     

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    2/16/2025 6:20 AM  #10

    John Ha
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    From: Mitchell, SD
    Registered: 4/02/2013
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    Re: Brake light switch and intermittent brake light operation.

    moon4964 wrote:

    My 1964 Falcon uses the brake light switch operated by brake fluid pressure not the mechanical pedal-mounted type.
     

    Wondering if you ever got this figured out .... just curious.

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    2/16/2025 5:18 PM  #11

    moon4964
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    Registered: 9/04/2024
    Posts: 46

    Re: Brake light switch and intermittent brake light operation.

    I will have more news on this problem this week.

       •Thread Starter
     

     

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