Datsun Roadster Forum-311s.org

If you have a 68-70 you of course have one of these. Not sure how many of you even care about this but I was curious. As I have built harnesses I wondered how this thing works. The parts manual exploded view didn't give me the detail I wanted. I got one to take apart.

Exploded view photo.

The top of the valve has the contact pin the wire hooks to. It's insulated and the tip inserts in to the body where the pressure differential piston is. So based on what I can understand is when there is too much difference between the front and rear pressure it slides to one side or the other which then touches the tip and grounds the circuit turning on the dash light.


This is the passenger side of the valve. You can see here the pressure contact piston that slides back and forth. Made of steel with two seals. The tip sits in the middle of this. Had a hard time getting it out due to being frozen. So I doubt any of these things still work.


The driver side of the valve.


This was attached to it also and I don't remember which port. I have no idea what is.

Curtis, thanks for putting this up. I’ve no doubt that most of these switches are frozen.

I think the unknown item might be an aftermarket pressure regulator valve to adjust the front/rear brake bias. I’ve seen similar looking devices from decades gone by.

Peace,
Pat

It can be nearly impossible to get the caps to seal after taking apart , also the Piston to slide back in & work freely as very close tollerance ....so best to replace
If someone is thinking of rebuilding
They are assembled submerged in fluid come capped full
It's a antiquated component really just to help you make it the 50 miles to.a dealer like in the early 60s on a single MC it shuts off the leak side

Replace with what? They're NLA and I wouldn't be surprised if there isn't even one NOS one out there. From what I see you're better off to leave the piston frozen and forget it. Most people have me use the S brake switch for something else.

Has anyone out there ever had this thing work and tell you there was a problem?

Just stating what tech training said but, I've taken 2 apart that did not go back , despite trying ,functional
Aside from retaining fluid if a line brakes in 1side (which your likely to realize a line broke quickly...... hence it's called the Idiot swt.) it's not all that needed
Many salvage yards remove the safety swt. and will not sell it same for MC s

Replace with one off almost any same size model car it's a vender company product or was back then

Curtis wrote: Sun Feb 27, 2022 10:52 pm
Has anyone out there ever had this thing work and tell you there was a problem?

I have had it work on my 69 SRL. I had run stainless steel fuel lines from a dual port fuel regulator to my Solex carbs. I made a bracket to keep the lines away from wires and brake lines and such. The bracket broke at some point allowed the steel lines to rub on the brake line to the proportioning valve at the firewall...driving one day, going fast I decided to pull into a local automotive service station to show the owner friend of mine the car ...step on the brakes...no pedal, S brake light came on...oh crap...ditched the idea of turning and limped home using engine brake. The fuel line had rubbed a hole in the brake line, brake fluid everywhere...

Has anyone checked with Karp’s to see if they can repair these? I have a 67.5 so don’t have this valve, only a block.

Peace,
Pat

Curtis wrote: Sun Feb 27, 2022 10:52 pm … Has anyone out there ever had this thing work and tell you there was a problem?

Mine comes on as I’m bleeding the brakes. It goes off as soon as I retighten each bleed valve.
I guess that means it’s working

Nice write-up & pics BTW Curtis

I have my switch (68) apart and have cleaned it thoroughly. Yes, it was stuck. I am going to attempt to replace the o-rings and hope that it works. Assuming, as this thread says, it probably will not work (leak-by or continue to stick). If I understand it correctly, its only function is the warning light, and does not serve as a proportioning valve, though i see how it could equalize pressure between front and rear. Is there any reason to keep the switch, or should I just eliminate it when I run my new brake lines? Another option might be installing an aftermarket proportioning valve. What is the typical resolution for this part?
Thanks for the pics and write-up. I thought I knew how it was assembled and worked, but your information confirmed my suspicions!

It's a brake safety swt.
Once apon a time (late 50s 60s) when master cyl.s were
Single ,if a brake line broke ,leaked you would soon have 0 hydro braking
and back then there wasn't 50 dealer in a state, try 2
a mom and her kids driving a hilly desolate road ...get the picture ? But in present day they are somewhat obsolete, except see how the swt works does it lite unplugged ,I can't remember , so yeah you could remove it
When the valve is assembled at the factory its done submerged in brake fluid as getting the air out of the side hydro chambers is nearly impossible with the .0002 clearance on the sliding shaft to body . on the early style
datsun didn't make a kit to rebuild it when I was there...it's a vendor supplied part

Curtis, Nice pic's and write-up, I always wondered.
Steve_69, Spriso's write-up on his very attractive brake line re-routing includes a complete explanation of the valve, purpose and perspective regarding its usefulness, well worth a read.

Here you go:
viewtopic.php?f=2&t=11485

Great. Thanks for the link. That is an outstanding thread for sure. I must have skipped over that one because I thought it was about modifying braking system for an engine swap. I am going to take the route of eliminating the switch and clean up the routing. For what it is worth, I am using the Eastwood tool. It is very good for this task.
-Steve