Brake Light Switch

[PT Moe]

Still working on my '41 Plymouth truck. Only been about 50 years, so I'm getting there. I did the front disc brake conversion with the upgraded duel master cylinder from DCM (Toyota?) and the PV 2 proportioning valve that I see some people on here have used. I was wondering what people have done for the brake light switch. Either inline hydraulic or will the single wire on top of the prop valve work. I do have the cap with the wire that's not shown in the picture.  I did talk to DCM and he said the plug on the side of the MC will not work. he also said the front port on the MC should go to the rear brakes. Wondering if it matters with a Prop valve in the line up. Thanks for any insight. Jeff  

[Young Ed]

First of all good job on the truck you're ahead of me on mine. I got distracted and went for a 47 Dodge truck that was easier ..... 

There are some mopars that have the brake light switch further downstream in the system usually at a T. I think Dad's 54 Plymouth is like that. 

[Los_Control]

What I did is go with a electric switch. 1949 chebby truck $12

 

 

While it may not be a issue .... some complain that if you just lightly use your brakes in traffic ... the hydraulic switch will not activate the lights, warning the driver behind you.

Mechanical activates as soon as you move the pedal ...

Just my feelings on it.

 

No idea what the wires are for on top of the MC ... I have a Toyota MC also ... I'm thinking possibly a idiot light for low brake fluid or such.

For me electric switch was just easier to wire in my add on turn signal switch ... brake lights go through the turn signal switch ... just simple instructions to follow.

Edited April 11 by Los_Control

[LazyK]

The connection on the prop valve is not a brake light switch. I believe it is used to warn if there is a imbalance/failure in one of the systems

[kencombs]

  On 4/11/2025 at 8:53 PM, LazyK said:

The connection on the prop valve is not a brake light switch. I believe it is used to warn if there is a imbalance/failure in one of the systems

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Correct!  It closes when the piston inside moves toward the side with low pressure.

 

Los has the right idea IMHO.   I've changed more than one leaking hydraulic switch, and  all the new ones I've seen recently are imported.  Thus more likely than older ones to fail I'd also go with the mechanical switch.   I don't want anything that could cause a loss of fluid or allow air in when there are alternatives.

 

Probably wouldn't change a stock switch, but since mine is getting hanging pedals, master and all new plumbing it is getting a mechanical switch.

[PT Moe]

Thanks for the replies. Good to know the cap in the center of the prop valve will not work. I could do the inline hydraulic switch but I think I'll see about configuring the mechanical switch before I cut back into my lines.  

[Los_Control]

The other bonus .... it's cheaper and less headache.

 

 

Just assuming you have 5/16" line, you need a T, then a adapter to drop it down to 1/4" line .... then another adapter to convert it to the same threads as a pressure switch.

Depending on if you have a original switch or aftermarket ... thread pitch is different then standard brake lines ... so you have some converting to get it to where you need it.

 

I'm just suggesting, there is probably $25 worth of fittings in the photo, plus another $25 for the switch .... a headache to put it together .... A $15 mechanical switch makes all those issues go away.

 

I still say a working mechanical switch is safer then a pressure switch ... if you are just creeping along in traffic and it is possible you are only applying 3psi pressure to the switch .... but it requires 5psi to actually activate the brakes lights .... yeah they are cool and a thing of the past, there is a reason they are not typically used today.

[PT Moe]

Is the mechanical switch activated as you push the pedal and the pedal shaft moves away from the arm on the switch? Seems to make more sense that the other way around. My cab is still at the painters shop so I'll have to figure it all out when I get it back on the frame.

Cab in primer

IMG_9420.heicIMG_9420.heicFetching info...

[Los_Control]

I would have to look at it again, certain my switch is mounted to the toe boards of the truck, underneath.

 

When the brake pedal is moved, the switch is activated.

Some old trucks you may have to move the pedal 3" before it gets any pressure and activates the lights .... some you move 3" and it activates the brakes.

 

The problem arises when in stop and go traffic and you have a manual transmission .... most people could manage to crawl along and never activate the brake lights ... with a pressure switch.

 

With a mechanical switch, as soon as you move the brake pedal ... the switch activates the lights .... warning the driver behind you we are riding brakes and not going anywhere.

The pedal movement may not even be strong enough to activate the hydraulic brakes ... it is enough to warn the driver behind you we are not going anywhere soon.

 

The same situation with a pressure switch .... driver behind you may never ever see a brake light and wonder .... I'm just saying.

 

These old cars ran for 75+ years with a pressure switch .... it is period correct .... given a choice I would not.

[kencombs]

The mechanical switch needs to be installed so that it  is 'open'  when the brake pedal is at the top of it's travel.   That position will push the switches lever to open the contacts.   A very short movement will release the lever and close the contacts.   In my case it's much easier as my switch will be inside the cab mounted to the bracket that holds my new, used, hanging pedals.   They came from a 60 model pickup and were a bolt in, after adding the factory reinforcement plate and drilling some holes.

 

One could use a normal push button switch, like often used for door operated interior lights also.   Just depends on the mounting, space,location etc.   It is nice to have it inside , but not necessary.   If exposed to the elements more attention is needed to switch quality to prevent corrosion.

[bkahler]

I just used a Tee off the rear brake line connection to the proportional valve and plugged one leg of the Tee.  Seems to work just fine and I'm using Dot 5 brake fluid.