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Question on early S30 brake pressure warning device


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In reassembling my, now, 50-year-old, early '72, HLS30-46372, build date 9/71, I've run across an issue with this device, which is supposed to warn if there is a significant pressure differential between the front and rear brakes.  I cleaned up the part and noted that while I can see daylight looking into the front brake line inlet (air will easily flow in that port and then out the other two front brake line ports), I cannot see daylight looking into the rear inlet port.

I don’t know if there’s simply gunk clogging the channel or whether there’s something more substantial blocking it (the piston?).  To compound my problem, my device looks nothing like the illustration shown in my 1972 FSM.



I am able to remove the electrical switch as well as the nut gland on the forward portion of the cast aluminum body.  I am, however, reluctant to start picking and prodding the internals until I get whatever guidance you more learned folks might have to offer.  Heck, a cutaway of the actual device that I have in my hands would be helpful at this point.

Frankly, because my car has rear disc brakes, I’m inclined to get a Wilwood proportioning valve to replace the whole thing.

Wilwood valve.jpg

I’m a little leery of how much reworking of the existing hard brake lines that will necessitate.  I don’t mind fabricating new lines from the master cylinder to the Wilwood, but what all would I have to do to the “out” lines?  And what do I do about the "valve" in the rear of the car?

TIA for any and all experiences and opinions you folks can provide on both items: (1) existing assembly issue and (2) a Wilwood conversion.


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I have two '72 cars: numbers 46372 (9/71 build) and 81416 (5/72 build).  The former has this device and the latter has the double gland nut device you referenced.

I think my older car was made with a lot of leftovers from the '71 model year as it must have been on the cusp of the switchover.  There's this device and then the rear window glass has the '71 defroster grid.

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Thanks for that reserach.  Yeah, I did see and read that thread before I posted this one.  Kind of hoping that someone had already worked out how to address the stuck piston issue as I'm fairly confident that is my problem.  The part is soaking in Kroil right now and I'll see in a couple of days if that has any effect.  Unfortunately, if the piston is stuck, it's stuck at the end of the bore where I can't break it free with the tap of a drift pin.  That's why I'm looking at the Wilwood valve.

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The wilwood does something totally different.

The nissan part senses a circuit failure front or back. Slides the piston and lights the brake light.

The wildwood changes brake bias front and tear which is what the part on the rear circuit is supposed to do.

I wood heat the aluminum to get the caps off. The aluminum should expand faster that the steel. Good fitting impact sockets that wont expand. One I had them loose. I would try to block the ports as necessary and use air to move the piston.

Failing that I would rig up a way to use a grease gun on it and see if I could force it out that way.

I wonder if a PO had a circuit failure and replaced the switch on a misdiagnosis and the second switch did the same thing.

You could also source a new switch with the double end caps

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That type of switch was used on many different brands of car.  You could probably find one and plumb it in as a surprise for a future owner.  That might be fun and it would be a new part.  I posted a Ford thing below. 

I had a pressure differential problem when I first got my car.  The light stayed on but it was pretty obvious my front brakes had low pressure because the pedal went to the floor and the front brakes didn't work (calipers on the wrong sides, big bubbles).  After the ordeal I realized that the switch and the light really didn't add much value.  You could just get rid of it entirely and be just fine.

If the seals don't seal you might also get fluid transfer from one end to the other.  Another thought.


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Thanks to all who have contributed so far to this topic.

To reply to Patcon: It seems to me that the switch on my car - and I'm the original owner - is not meant to be serviceable.  At least it seems that way to me as I have no idea how to "dig out" the male flare fitting buried at the open end of the main chamber.  I've never had to do such and extraction BUT it would be a good talent to have, I think.  I wonder if anyone here can shed some light on such a procedure?

I'm leaning more and more towards getting that Wilwood unit and then finding the "sweet spot" so that my front and rear discs lock up simultaneously.  I was initially concerned about the proportioning valve on the rear brake line by the gas tank, but have since been convinced from other threads that it is simply a rear brake engagement delay device and not a pressure regulator per se and so, with a Wilwood unit up front, can be deactivated by removing its guts so it functions only as a T-connector.  Is that how you interpret its function?

To reply to Zed Head: Thanks for the Fordification link.  From the cutaway of the Ford unit, it appears to have something akin to a set screw at the back end so one could insert a rod to mechanically reset the piston should it ever get seriously displaced.  As several threads on this topic have mentioned, without springs at both ends there is nothing to reset the piston once any significant differential pressure issue gets resolved.

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Mine was full of crud and slim. The piston aas not moving freely so I dismantled, cleaned it and replaced the o-rings. It has worked fine ever since, all 5 years.

To get the cone section ouf the end, I heated the outer aluminiun body with a heat gun and tapped it hard on a wooden block. After tens minutss of tapping, heating and tapping it came free. 







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