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1972 Z Brake switch / valve


JLPurcell

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2 hours ago, Zed Head said:

On top of that, add that it doesn't really make sense to have to reset the switch, since it moves by pressure differential

Not so sure about that.  Obviously the movable valve/plunger/whatever will shift off its normal, centred position if hydraulic pressure drops on the left or the right side.  And once it shifts off centre, it stays there.  So let's say you fix the problem you had in the front or rear brake circuit.  Now when you step on the brake pedal, the valve/plunger once again sees equal pressure on both sides -- so it won't move from its new, off-centre position.  I'm inclined to agree with Grannyknot that it needs to be manually re-centred.  Can't verify from actual experience, but, hydraulically speaking, that's the way I see it.  Unless someone can come up with an alternative explanation.

A couple of other comments:

I disassembled the switch from my 4-70 car and it was kind of gummed up.  But probably not so much that it wouldn't have shifted had I lost pressure in one of the brake circuits so that the valve/plunger was exposed to full pressure from one side only.  That's a lot of force.

I think the factory didn'y want anyone messing around with this switch because an internal leak caused by a botched service job could result in the front and rear brake circuits becoming connected, thereby negating the safety function of the dual-circuit braking system.

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2 hours ago, Zed Head said:

image.png

It doesn't stay there.  That's my point.  There's nothing holding it there, by design.  If you fix the problem and restore the proper pressure to the other side of the switch, the switch moves back to its proper centered position.  Like Nissan says.

Edited by Zed Head
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1 hour ago, Zed Head said:

the switch moves back to its proper centered position.  Like Nissan says.

It can only move when there is unequal pressure in one of the 2 lines or when it is reset manually.

 

1 hour ago, Namerow said:

And once it shifts off centre, it stays there.  So let's say you fix the problem you had in the front or rear brake circuit.  Now when you step on the brake pedal, the valve/plunger once again sees equal pressure on both sides -- so it won't move from its new, off-centre position.  

This

Also, Nissan doesn't want anyone messing with the switch, that is just the small screw in plug on top of the brake failure warning unit. The switch itself is easily broken, I went through a couple.  Maybe the later ones self correct but I'm talking about the early units.

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I see the logic now.  They might have added a self-centering mechanism that didn't require a pressure differential in the later revision.  Maybe the ramped portion of the valve plunger.  Still unclear to me though that the original 72 switch, in good shape, required a reset.  Just haven't seen the firm example.  Even your anecdote is about broken switches but nothing about resetting a stuck one.  If I had one, I'd test it and see what happens.  If you can reset it with a small screwdriver you should be able to get it stuck with one.

Not really arguing just looking for some solid evidence.

Here's the 72 and 76 switches, you can see the ramped area in the 76.  It looks like the 72 has a spring to reposition the valve, but it's not clear.   The 76 switch might even work in a 72 but then you'd have to wonder about the VIN.  Just kidding.

image.pngimage.png

image.png

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1 hour ago, Patcon said:

Zed

Your first two images both show springs on both sides of the valve. So you would think it would self center, but as gunky as these can get, who knows. I have never disabled one...

I left two of the same image in the post as an act of rebellion against the wonky post editing software.  But, yes, those are the springs I thought I saw.  Maybe the switch tended to get stuck and that's why redesigned it.  There's not much to it, no reason to redesign unless there was a problem.  Functions are identical.

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1 hour ago, zKars said:

Has anyone, anywhere, any time ever had a front or rear circuit fail and have that light actually come on? 

My 1976 switch and light worked fine, many times, as I tried to bleed my front brakes in the days after I got my car.  Turned out the calipers were on the wrong sides, upside down, and there were big bubbles in both.  If I recall correctly, pumping the pedal would cause the light to go out and the front brakes to work.  The workout probably added many miles and years to its life.

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I have had the light come on (same as the handle brake light on my early 71) when the brake fluid was low enough to need to be filled and bled. I am not sure why but I have always believed that meant the brake system needs attention. Also it seems to have an brightness intensity as it progressively got worse. Or maybe it happened while driver was “Impaired”.

 

 

Sent from my iPhone using Classic Zcar Club mobile

 

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On my early 71 my rear wheel cylinders were frozen which would cause the light to turn on when the brakes were applied. As soon as the pedal was released the light would go out. After replacing the wheel cylinders and bleeding the whole system the light now only comes on when the parking brake is applied.

My FSM purchased with the car in 71 shows the springs in the sensor switch just like the BR-14 illustration posted earlier in this thread with the same warning about NOT servicing the unit. So I think all the Z cars have self centering sensor switches.

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22 hours ago, Patcon said:

Zed

Your first two images both show springs on both sides of the valve

SPRINGS... so that's what all that rust was, when I took my brake warning unit apart there was a lot rust and brown mayonnaise stuff, I thought the rust was from the piston but it seems now it was what used to be springs. Well then ZH   I concede, sorry for confusing the thread. That illustration BR-14 clearly shows them.

Edited by grannyknot
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On 11/26/2017 at 6:27 PM, Zed Head said:

I see the logic now.  They might have added a self-centering mechanism that didn't require a pressure differential in the later revision.

ZH wins the debate.  I forgot all about those springs.  And they weren't a later add-on.  Here are pictures of the internals as they came out of the unit installed in my April 1970 Z...102_4437.JPG102_4441.JPG

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