1972 Z Brake switch / valve

[zKars]

Yup, looks like mine.

Notes.  

Both sides are identical, save for the hole in the one end cap where the 2nd front brake line goes. The center piston is symetrical as well. The center piston moves very easily and would take only a couple of psi differential to move it, other than the spring pressure. They are pretty stiff.

The center electrode chamber and center of the dumb bell shuffle piston never sees brake fluid. 

The threads on the electrode retaining cap are not sealed to the outside world so moisture can get in and corrode. There is an o-ring at the base of the electrode that sticks into the dumb bell gap that keeps the outside world out of center of the dumb bell shuffle piston. 

The copper tip of the electrode needs to be clean enough to conduct when the shuffle piston touches it. Both the two I took apart were not dry or clean in that cavity.

It's easy to damage the electrode ceramic coating when you take out the center retaining cap. It is supposed to be free, but its often corroded in place, as it has no seal to the outside. As you remove it, the electrode has to rotate along with the retaining cap or the ceramic may crack if the cap is coming off while the electrode stays in place.  Gentle heat and penetrating oil and patience are a must here unless you're lucky.

A bit of cleaning and two new orings on the shuffle piston are all it needs to work again if the bores aren't pitted.

 

Unless you're doing a resto where stock appearance and function are important, I'd be doing this part instead to replace both this distribution block and the stock rear (or front, later) prop valve. Nothing worse than old brake parts. Ok, maybe old seat belts...

[grannyknot]

25 minutes ago, zKars said:

The center piston moves very easily and would take only a couple of psi differential to move it, other than the spring pressure.

In a perfect world I had to soak mine overnight in thinners then spend a couple of hours trying to get the piston to move at all.

[zKars]

1 minute ago, grannyknot said:

In a perfect world I had to soak mine overnight in thinners then spend a couple of hours trying to get the piston to move at all.

No surprise. Water in the brake fluid would cause total havoc in there. Steel caps, springs and shuttle piston, aluminum case. 

Everyone get out there and flush your brake fluid, now! Okay, soon, like before next spring. 

[Captain Obvious]

The early design centers itself with springs. The later design centers itself hydraulically. I didn't know there was question about how the early design centered or I would have jumped in earlier. I'm no expert, but I could have helped there.

One question though... My engineering sense tells me the springs in the early design should be different on the two sides because of the proportioning valve. Are the two springs identical, or is one of them wound with thicker wire or have one more turn or something?

[zKars]

Both springs appear identical. Proportioning valves are down stream of this device, which compares the output of the two master cylinder circuits. 

Edited November 28, 2017 by zKars

[zKars]

And I finally realized there really are two different types! Given the crap I've seen in the early ones, I'm hoping the later ones better sealed.

[gundee]

On my bucket list is to remove this valve assy. totally from my system. I see no need.

I want separate piping direct to both front and rear. Next owner can do what he wants.

[Zed Head]

1 hour ago, Lumens said:

I see no need.

When I was having my caliper bleeding problem I thought the same thing.  The brake performance had to be terrible before the light came on.  The pedal dropped a couple of inches with no brake action.  Pedal action told the story well before the light came on.

[Captain Obvious]

10 hours ago, zKars said:

Proportioning valves are down stream of this device

Gotcha.

Looking at the designs, I can come up with one quick reason the later version is probably better... On the spring version, every time you let up on the pedal, the springs will re-center the piston and the light will go out. This means that every time you hit the brakes, you will have to waste fluid volume to push the piston to end of travel before you build pressure in the good side of the system. This is probably the explanation for the horrible pedal feel.

On the newer design, If I'm understanding the design correctly, the piston is centered hydraulically and will not re-center until the problem is resolved. In theory, this would result in one bad pedal push towards the floor, but after that, it should engage further up. All in theory, of course.

However, all that said, I'm starting to agree with the above which questions the need at all. I mean... I don't need a bright red light on the dash to tell me that my brakes have failed. I'll have a brown stain on my seat that will make the situation perfectly clear. A bright red light might even be a distraction at a time when I should be concentrating on other things, like plotting my escape strategy from impending doom in front of me.   

[Namerow]

31 minutes ago, Captain Obvious said:

Gotcha.

Looking at the designs, I can come up with one quick reason the later version is probably better... On the spring version, every time you let up on the pedal, the springs will re-center the piston and the light will go out. This means that every time you hit the brakes, you will have to waste fluid volume to push the piston to end of travel before you build pressure in the good side of the system. This is probably the explanation for the horrible pedal feel.

On the newer design, If I'm understanding the design correctly, the piston is centered hydraulically and will not re-center until the problem is resolved. In theory, this would result in one bad pedal push towards the floor, but after that, it should engage further up. All in theory, of course.

However, all that said, I'm starting to agree with the above which questions the need at all. I mean... I don't need a bright red light on the dash to tell me that my brakes have failed. I'll have a brown stain on my seat that will make the situation perfectly clear. A bright red light might even be a distraction at a time when I should be concentrating on other things, like plotting my escape strategy from impending doom in front of me.   

Well, it was a different time and place back then, wasn't it?  The concept of safety systems and warning lights was brand new (how many warning lights were there on the dash panel of a 55 Chevy?). We'd all been reading about Mercury and Gemini capsules and Mission Control  and supersonic aircraft and James Bond, and it seemed like there could be no such thing as too many warning lights. Or buzzers.  Or digital warning messages ('Your door is ajar.')

Of course, we're much smarter now.

[sweatybetty]

51 minutes ago, Namerow said:

Of course, we're much smarter now.

  thats what we think anyhow

[JLPurcell]

I am at wits end on the brakes. I have not started randomly replacing parts, but don't know how to continue in trying to diagnose my problem. A few months ago I found a set of NOS MK63 calipers (for vented rotors). They had been sitting for years. I found rebuild kits took them apart had them cleaned out and reassembled them. I found a set of vented rotors and installed the upgrade. My complete brake system was gone through in the restoration completed in 2012. The system worked well prior to the upgrade. The car has a stock 7" mastervac and TOKICO 7/8" master cylinder. Since installation I have a hard pedal. When I pump up the brakes and then start the car. the pedal moves just slightly. When I removed the hose to the booster is was holding vacuum. I have re-bleed the  brake numerous times with no change. I am ready to put the stock system back in place and drive the car! Anyone have any ideas on how to continue to look for the answer? Thanks JLP