Summershandy

Brake lights on again!

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When I got my '54 Pontiac, I noticed the brake lights stayed dimly on. No problem, as over the winter the brake system was getting replaced along with a new hydraulic brake light switch. All the rear lights work normal and perfect now. Last week I've been firing up the engine, getting it to temp and checking all gaskets and hoses for leaks. Today I pumped up the brakes and held them to check for any leaks in the lines. I came back to the garage later and found the brake lights on full bore. Surely this switch hasn't already failed? I'm assuming there's a pressure build up holding the switch on? Is there an adjustment for this? I see the threaded rod for the master brake cylinder. Is it at there? I can't believe I'm weeks away from getting the car certified for the road after 8 months and this is back to haunt me. 

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There were some posts about the newer brake fluid (silicone I think) ruining hydraulic brake light switches.  Could this be your problem or is the system holding pressure???

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Are you sure the MC piston seal is BEHIND the compensating port? If not it will store pressure.

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If it's holding pressure, then one or more wheels would be locked up? I could jack them up tomorrow and check for that?

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Expecting a 54 Pontiac to be wired like a 54 Chevy, I can only suggest disconnecting the brake switch to eliminate a possible steering column / turn signal or harness short before tearing  the brakes apart. 

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The brake pedal feels fine now like it always did. I gave it a couple taps with a hammer thinking maybe it would back off. I disconnected the switch and the lights went out. I've heard of these switches only lasting 6 - 12 months but I'd say I've pumped the brake pedal maybe a dozen times in the switches life. Only a matter of time when others chime in with Echlin SL128 switch. I wanted to stay completely original but.......

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How about the brake return spring, whose attachment hole is visible in your photo of the as-yet-unrestored pedal and master cylinder assembly?

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1 hour ago, Grimy said:

How about the brake return spring, whose attachment hole is visible in your photo of the as-yet-unrestored pedal and master cylinder assembly?

I was thinking that, too. Maybe the pedal return spring weakened.

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Right, specifically the PEDAL return spring.  Thanks, John.

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There is a relief in the master cylinder piston. The MC rod needs to be adjusted so that, at rest the piston comes back far enough to clear the compensating port in the fluid reservoir.

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9 hours ago, keiser31 said:

I was thinking that, too. Maybe the pedal return spring weakened.

Spring is good. There is no play in the pedal. I gave it a good pull thinking the lights may go out. Which adjustment is it? Do I remove the bracket at the pedal and turn there? Seems that would only change the pedal to floor height. Or is it the nut (which isn't your typical wrench type nut) that's against the boot? My manual doesn't make it clear nor a picture.

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 Pressure switches usually fail in the off position, less likely to fail in on. 

 

However, there's a very simple way to test the switch. Disconnect the switch wires and hook a volt/ohm meter to it. Put the meter on the ohm scale and touch the meter probes to the switch contacts and see if there is a reading higher than zero. That indicates the switch is making contact.

 

 If  no reading,  then more likely the master cylinder piston cup is not moving back far enough to be just beyond the VERY tiny hole of the compensating port when the pedal is released and allow the pressure to bleed back into the brake fluid reservoir. The link rod needs to be adjusted shorter so that the piston retracts further in the bore just barely enough to uncover the compensating port opening inside the bore.  Sometimes that port is so small (sometimes as small as .025 inch), or plugged with crude, that many brake system novices aren't aware it's there.

 

If you can move the pedal and piston further back by pulling on the pedal then it's a too-weak return spring. If there's a pedal stop and you can get the lights to go off by pulling on the link rod, then the stop is adjusted wrong, or if not adjustable, there's slop in the pedal and link rod pivot connections.

 

Paul

Edited by PFitz (see edit history)

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Thanks Paul. I figured out how to adjust the push rod. Loosen the lock nut and turn the rod. I took up a little slack but then all it does is adjust the pedal height once the piston is all the way out. Lights stayed on. I had forgotten my multi meter. I removed the wires and got a reading from the switch. So the switch is indeed on? There should be no crud in the systems as it's completely new and the switch hasn't even seen the road yet! Talk about crap. Wonder what made it fail so soon? 

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Here is your test.

When its acting up crack one bleeder. If the lights stay on its the switch.

If the lights go off it is likely the master cylinder.

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Got'cha...see if pressure is holding switch on. Cracked a bleeder and lights stayed on. Better a switch than a new MC. Thanks for the tip JACK M!

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9 hours ago, Summershandy said:

Thanks Paul. I figured out how to adjust the push rod. Loosen the lock nut and turn the rod. I took up a little slack but then all it does is adjust the pedal height once the piston is all the way out. Lights stayed on. I had forgotten my multi meter. I removed the wires and got a reading from the switch. So the switch is indeed on? There should be no crud in the systems as it's completely new and the switch hasn't even seen the road yet! Talk about crap. Wonder what made it fail so soon? 

 

With the switch wires disconnected, and the ohm meter hooked up to the switch contacts, if the meter needle moves off zero to show a reading, then the switch is shorted internally.  Time for a trip to the auto parts or motorcycle shop to get a new pressure switch. Looking at the one in your picture NAPA has them - Echlin #113. Same as used in Wagner Lockheed systems since 1928. 

 

And as to the "rumor" of not using silicon brake fluid...... In the new Echlin 113 switch I bought at NAPA last December, was a factory note saying do not use DOT 5 fluid with that switch.  No explanation on the note why that is.   

 

Paul

Edited by PFitz (see edit history)

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12 hours ago, PFitz said:

Put the meter on the ohm scale and touch the meter probes to the switch contacts and see if there is a reading higher than zero. That indicates the switch is making contact.

 

???

 

Zero ohms is a perfect connection. If a switch reads zero ohms between the terminals, then the contacts inside are "made up" and current will flow.

 

When an ohmmeter  has leads in mid air, the resistance between the leads is infinite. Old analog meters actually had the infinity symbol at the end of the scale opposite the zero (0) ohms.

 

Modern DMMs show 0.0 (zero) when there is no resistance, i.e. best possible connection. Typically with the leads in place, touching them together gives 0.1 , 0.2 ohms. With leads in mid air Fluke's show OL, for open leads, which in infinite ohms.

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I usually use the "beep" function on my VOM when doing this kind of stuff.

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The return spring on the linkage SHOULD be holding the pedal in the full up (off) position. Fix this first!

Nothing else in the system holds the pedal up and away from the master cylinder. It is a meaty spring. 

 

To check the push rod adjustment on the compensating port: 

Remove the cap on the master cylinder. look into the reservoir AS YOU PUSH THE PEDAL. 

There should be a small squirt that appears within the fluid, that is in the reservoir, each time you push the pedal. (if you push real hard or slam it, you can even get it to splash out of the reservoir) 

If there is no squirt/splash seen - SHORTEN the push rod slightly, and try again. The squirt should consistently happen within the first inch or so of pedal travel. 

 

The rod adjusts how much pedal travel is needed to move the piston into the MC bore. There are TWO holes in the MC (you can see both from the top) The big hole is the intake hole it allows fluid to move into the piston chamber.  The smaller hole (which should have been verified as not being plugged during the rebuild) allows fluid to move back into the reservoir and RELEASE the pressure as the pedal returns. In the reverse action NO PRESSURE is created in the brake lines UNTIL the piston moves forward and covers this small hole. Then the pedal action can pressurize the system. 

 

The rod IS NOT to be used to adjust pedal height!

The rod does NOT compensate for improper adjustment at the shoes/drums. 

 

Edited by m-mman (see edit history)

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21 hours ago, Frank DuVal said:

 

???

 

Zero ohms is a perfect connection. If a switch reads zero ohms between the terminals, then the contacts inside are "made up" and current will flow.

 

When an ohmmeter  has leads in mid air, the resistance between the leads is infinite. Old analog meters actually had the infinity symbol at the end of the scale opposite the zero (0) ohms.

 

Modern DMMs show 0.0 (zero) when there is no resistance, i.e. best possible connection. Typically with the leads in place, touching them together gives 0.1 , 0.2 ohms. With leads in mid air Fluke's show OL, for open leads, which in infinite ohms.

 

Yes, moves meter reading off zero, even if making contact. There's the world of new, perfect connects,..... and then there's the world of old car connections.

 

Old brake switches, even the best of those that work, do not have "perfect" contacts - they get cruddy with age. So, they will show some amount of resistance reading when the contact diaphragm closes across the contacts under pressure. If they have no pressure and show a reading (or brake lights on) the switch contacts are shorted to each other.

 

 When doing an ohm reading, zero an analogue meter's needle.  Then touch the probes to the switch contacts and the needle will move off zero to show a reading of the resistance to the flow of the low battery voltage of the meter through those less than perfect contacts. Digital meters will move off 00.0, or OL  (open line), and show a number readout for the resistance through the switch.

 

For those interested here's pictures of the new NAPA Echlin SL113 switch and the note that comes in the box with it that warns against using silicone brake fluid.  

 

Paul

DSCN0020.JPG

DSCN0021.JPG

Edited by PFitz (see edit history)

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I have been using NAPA Echlin brake pressure switches specifically SL142 for years with silicone fluid with no deleterious effects. Silicone brake fluid will effect old NOS brake light switches.  The little white slip is not in the box nor is there anything on-line about it that I can find.  Possibly the SL113 is internally different. 

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11 hours ago, PFitz said:

Digital meters will move off 00.0, or OL  (open line), and show a number readout for the resistance through the switch.

Hopefully my new switch comes in today. I will pull the old one out and test both on my MM. I'll try to get the new one in and confirm the brake lights.

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14 hours ago, m-mman said:

The rod IS NOT to be used to adjust pedal height!

For the 1954 Pontiac, the MC push rod is used to adjust the pedal height to the required 4-3/8" from the floor mat. I observed the action of the ports while bleeding the system as you mentioned. Thanks for the suggestions!

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