'40 brake lamp switch / master cylinder issue?
 

Okay guys, I'm getting frustrated.
Working on a '40, stock brakes.
Wiring is all good for certain.
I can get the tail lamps to light up IF I jump the two wires at the brake lamp switch.
I have power on one wire as I should have.
I have replaced the switch three times. Two nos switches and one brand new replacement.
As you guys know, when applying the brake pedal, the hydraulic force should close the switch circuit and the brake lamps should light up. Testing, there should be power to both terminals at the switch.
They don't. Not with any of the switches.
So, my way of thinking is the hydraulic pressure is not activating the switch(s).
Car stops just fine and has decent pedal.

Oh, if I jump the two leads at the switch, the brake lamps are nice and bright.
What the heck am I missing?

Re: '40 brake lamp switch / master cylinder issue?
 

An obstruction in the master cylinder port?

Re: '40 brake lamp switch / master cylinder issue?
 

That's my thinking Denny. If I pull the switch with the pedal engaged, I do get fluid to come out of the rear of the cylinder. So, at least I know it is not fully obstructed.

Re: '40 brake lamp switch / master cylinder issue?
 

Mike, unless I'm missing something, it can only be 3 duff switches. If you bypass the switch, the brake lights work. Do you have a known good switch on another vehicle? Try fitting that just to prove the point.

Is the switch fitted to the normal 40 type banjo bolt? If so there is no way it can not be seeing pressure if the brakes are working. Is the banjo bolt new of previously used? Is it drilled correctly?

If you loosen the switch slightly and pump the pedal, does fluid leak past?

Re: '40 brake lamp switch / master cylinder issue?
 

I would swap out the MC. Maybe the fluid you see coming out the back isn't sufficient pressure to cause the switch to operate.
I would also try to operate the switch with air pressure and see if the lights come on.

Re: '40 brake lamp switch / master cylinder issue?
 

I did order a new master cylinder from Napa earlier today. I have had good luck with those in the past.
The brake lamps did work a few weeks ago albeit only for a short time. A person following the car had said the lights seemed to "fade" away.

Mart, all authentic brake parts. I agree with you that three switches could be bad. However, at this point, I'm attempting to avoid any more frustration than necessary. The new master is only about $75.
I'll do my best to install it Thursday and will report back.

Gee, and I thought I'd seen it all :)

Re: '40 brake lamp switch / master cylinder issue?
 

Silicone brake fluid? You need a low-pressure brake light switch for DOT 5 silicone fluid compatibility...


https://www.summitracing.com/parts/rfw-sw32

Re: '40 brake lamp switch / master cylinder issue?
 

To quote an old baseball saying : "Say it ain't so, Kube.".:D

Re: '40 brake lamp switch / master cylinder issue?
 

Kube never even hinted at SILICONE brake fluid! Where the heck did that come from?

Coop


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Re: '40 brake lamp switch / master cylinder issue?
 

bad stop light switches. Check them out on your air compressor using an ohmmeter. Should have very good continuity approaching zero ohms.

Re: '40 brake lamp switch / master cylinder issue?
 

On my 40 I had no Brake lights. Test light showed power to the Switch. Still did not work. Dug out a Good Meter, Only 3 volts going to switch. (?) Not enough power to light Brake lights. Still do not know why. Ran a new wire to the Switch with full 6 volts. Brake light work now.

Re: '40 brake lamp switch / master cylinder issue?
 

If I cross the terminals at the switch, both lamps are very bright.

Re: '40 brake lamp switch / master cylinder issue?
 

Is there a turn signal switch involved in the circuit?

Re: '40 brake lamp switch / master cylinder issue?
 

Yes. However, they have worked for quite some time. And to reiterate, I have power TO the switch, just not on the opposite side of the switch.

Re: '40 brake lamp switch / master cylinder issue?
 

I have never taken a master cylinder switch apart but, is it possible there is an internal failure of some type? I am using a Harley Davidson brake light switch with no problems. Brake fluid is silicone.
Here's some reading about the HD brake light switch that might help.
https://www.fordbarn.com/forum/showt...k+light+switch

This is crazy idea but, could it be the brake fluid?

Re: '40 brake lamp switch / master cylinder issue?
 

I won't be able to swap the cylinder until tomorrow. My fingers are crossed that I am on the right path to a proper repair. I promise to let you guys know the outcome.

Re: '40 brake lamp switch / master cylinder issue?
 

Hi Kube,
Just my opinion, but there is no way that the master cylinder is bad if it has enough pressure to stop the car ( which is by far more pressure) than it is required to activate the stop light switch. There has to be a blockage / restriction in the fitting or bad switches. Just my 2 cents. Steve

Re: '40 brake lamp switch / master cylinder issue?
 

Mike
Did you connect an air pressure gauge to rear of mas cyl and check pressure

when pedal is held down?
Larry

Re: '40 brake lamp switch / master cylinder issue?
 

No, I did not. A grand idea indeed but I didn't think of that.

Re: '40 brake lamp switch / master cylinder issue?
 

Tonight, or most likely tomorrow, will tell the story :)

Re: '40 brake lamp switch / master cylinder issue?
 

Did you try wiring a temporary mechanical switch to see what happens. But if you say the lights work if you connect the teminals. I think it is a switch issue. I use the regular stop light switches, I can get 4 for the price if a H-D one and they work just as well.

Re: '40 brake lamp switch / master cylinder issue?
 

What am "I" missing. Seems every year a new brake switch. This month got one from O'rielys instead of Napa./ not to mention bad condensers and other irritating new parts. as a side note I have been watching mass production in China and I am impressed with factory's hospital clean conditions and space age measuring that makes micrometers a thing of the past. interesting.

Re: '40 brake lamp switch / master cylinder issue?
 

i gave up on hyd switches after 3 failures in 2 weeks went mechanical no problems 5 years and counting

Re: '40 brake lamp switch / master cylinder issue?
 

I suspect three bad switches, and will be very interested in what the final investigation reports.
I purchased ten new pressure switches about four years ago. My 53 seems to have a switch fail about every two years. I'm not willing to cut into my new wiring to install a mechanical switch when the pressure switch is so easy for me to replace (Ford put the whole mess under the hood starting in 1952, way easier to work on than 1940 through 1951). Between replacing switches on my car and helping other people with failed switches on tours I am down to three spare switches.

Re: '40 brake lamp switch / master cylinder issue?
 

A follow up is due to all the nice folks that had offered me advice. This one had really frustrated me.
Turned out to be a couple of issues that kinda "muddied the waters".
Master cylinder was dirty as was the entire hydraulic system. So, I believe it was a good thing that this issue showed up. I flushed the entire system and replaced the master with a new unit from Napa. I have had very good luck with these. As a plus, the casting and cap are concourse correct for 1940.
Then there's the switches. Four bad in a row. ARGGHH! Are you kidding me?
Three NOS, concourse correct switches and a new one from Napa. The fifth one purchased off of Amazon for $10 worked like a charm.

So, thanks guys for the advice and your patience with me.

Re: '40 brake lamp switch / master cylinder issue?
 

WOW! Four bad switches in a row.
That's a sad commentary on the quality of today's replacement parts. Glad you solved the problem.

Re: '40 brake lamp switch / master cylinder issue?
 

When I restored my '51 in 1987, I removed the original switch, cleaned it up and re-installed it in the rebuilt master cylinder. Not a problem since. I have never had a bad original switch in almost 70 years, so I'm saving them.

Re: '40 brake lamp switch / master cylinder issue?
 

How did you clean it up?

Re: '40 brake lamp switch / master cylinder issue?
 

QUESTION: I would think that to "clean up" the switch you would have to take it apart.
Then the problem becomes reassembling it and sealing it so it doesn't leak under pressure. As I recall there is a small diaphragm inside the unit that flexes under pressure to "make and break" the flow of electricity to the tailights.

Re: '40 brake lamp switch / master cylinder issue?
 

I had a problem with the switch on my ‘46 Coupe, bought one from the Harley dealer, and no problem since. Not concours, but it works.

Re: '40 brake lamp switch / master cylinder issue?
 

Actually, I just used some solvent on a rag to wipe the dirt and oil off the exterior. I was only concerned with cosmetics because it had always worked.

Re: '40 brake lamp switch / master cylinder issue?
 

Poopy. I was hoping you had figured out a positive method of cleaning the interior.
NOS (concourse correct) switches are rare. I'd sure like to figure out a method.
One trusted fellow here on the Barn suggested cycling the switch numerous times. I'll try that and see what happens.

Re: '40 brake lamp switch / master cylinder issue?
 

Mike, all, would it be an idea to rig up some sort of device to test the switches? An old but serviceable master cylinder, a lever and a couple of fittings might allow at least a heads up of a bad switch. Someone suggested using air.. is that feasible? What psi is needed to make the contact in a good switch?
I must admit, I fitted a mech switch in my sedan after a couple of failed hyd switches. My pickup still has it's original hyd switch and still works.

Re: '40 brake lamp switch / master cylinder issue?
 

Mart, I did rig up an air pressurized testing system. I set the pressure at 23#. I was advised that may be quite low.

Re: '40 brake lamp switch / master cylinder issue?
 

I hope a Fordbarner will carefully take apart a "bad" switch and examine it to ascertain what actually causes the switch to fail.
I thinks it's because the mechanical seal between the internal contacts eventually leaks preventing proper electrical continuity.

Re: '40 brake lamp switch / master cylinder issue?
 

A quick google search shows Ron Francis makes a low psi one for 20-50 psi, where as a normal stock 60-120 psi. Of course we know the pressures to apply wheel cylinders can go upwards of 1000 to 2000 psi for harder braking.
Steve

Re: '40 brake lamp switch / master cylinder issue?
 

That's very helpful Steve. Thank you!

Re: '40 brake lamp switch / master cylinder issue?
 

Your welcome. I also read normal braking psi is 300-500 psi where as 600 psi and above is considered panic braking. Glad you got your lights working as its nothing any more frustrating than something like that.
Steve

Re: '40 brake lamp switch / master cylinder issue?
 

Do you happen to remember the NAPA part number for the master cylinder, and which Amazon switch you went with?


Kirk

Re: '40 brake lamp switch / master cylinder issue?
 

Amazon:
WQSING Hydraulic Rear Brake Light Switch DS-272191 Compatible with Harley Davidson Sportster Softail Touring Dyna Super Electra Glide Road King FXR Deluxe Fat Boy Low Rider Heritage 72023-51A
Napa:
Part #: NMC M1050, with my discount, about $70. This is a brand new unit.