'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


.

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 :)