Here is something I thought would be useful and something I did for my v8 3 years ago.....I took this from Hybridz: Here is a link to the original post http://forums.hybridz.org/showthread.ph ... ction+disc

Perhaps I should preface this with my reaction disk story. Don't worry, I'll get to the pictures.
I own a '72 240Z, and had experienced all of the reaction disk symptoms. The problem was that I didn't know such a thing as the reaction disk existed. I had a pedal that did very little until the end of its throw, and then it would lock up the brakes if pushed beyond 3/4 of the way to the floor. There was only a narrow, touchy window of weak braking before lockup.
I initiailly bled the brakes and replaced the master cylinder with a rebuilt '79-'81 280ZX unit to no avail. I then replaced the booster with one from a junkyard '72 240Z, but no help. A few months later I installed new pads and shoes, a rear spring kit, and added new rear wheel cylinders because mine were leaking. After adjusting the rear cylinders, and thorough bleeding, there was no noticeable change.

I had taken apart the old brake booster and found a mysterious rubber disk floating around inside it, but didn't think much of it. It fit nicely into a little pocket on the diaphragm anyway.

This is what the innards look like.
The rubber boot doesn't belong in there, it fits on the back side of the booster, through the firewall.
Ignore the voltage regulator. I'm pretty sure it doesn't belong in there either.


After having run out of things to replace I decided to activate the good 'ole search function on this site. Lo and behold, after reading through a few dozen posts I came to the conclusion that the reaction disk would be something to have a look at.

What follows are the pictures I took while assessing the damage.

This is what you see when you pull the MC away from the booster. First remove the metal clip and pull out the push rod/plate and seal assembly.
It might just reveal your worst fears.




The little rubber spacer shown here is the reaction disk. This photo was taken after it was fished out of the booster with long needle nosed pliers. This also shows the backside of the push rod/plate and seal assembly. Note that since the reaction disk is rubber it makes a dull noise if you shake the booster. It only took a minute of shaking and bumping to get it within reach of the pliers.




Next is a view of the reaction disk epoxied onto the back of the plunger. You can also see what the inside of the booster looks like without the reaction disk in place.
Just in case, I took measurements. The disk is 5.5mm thick, and has a 25mm diameter. As you can see, its diameter is exactly the same as the backside of the push rod.


The whole assembly now slides back into the booster, and the reaction disk/push rod assembly fits into its receptacle on the diaphragm. Next, the whole reaction disk/push rod/plate and seal assembly is held in place with the metal clip.

In trolling these forums I had also come upon a post by 240playtoy with a nicely done diagram.
http://forums.hybridz.org/showthread.php?t=112496
It shows the difference in depths between the 240Z and 280ZX master cylinder pistons.
Initially I adjusted the push rod 1/2 in. further out to compensate for having installed the 280zx MC (as per 240playtoy's instructions).
However, reinstalling everything proved impossible with it threaded out that far. I simply turned it back down with my 7mm wrench until there wasn't any more interference.

With everything bolted back up, now my brakes actually work, and they work very well. Needless to say, I'm glad to have a vehicles that stops rather than just contemplating the concept of turning inertia into friction and heat.

I hope this helps someone in the future.

Great Post, Clive, and very interesting. I recall reading a similar story in one of my old restoration manuals from years gone by, or maybe in Bryan Little's DATSUN Z GARAGE website. I'm now led to believe these little "hockey pucks" are prone to separating from their seats over time, and should be checked whenever doubt ariZes.

My question is how he managed to separate the two halves of the vacuum chamber? I read how it was supposed to be done in a Resto book, but when I got one on the bench it proved nearly impossible ~ opening it up would have wrecked it completely, and I would have ended up throwing things around the garage long before I got it open, anyway!

Do you peel back all those little ears and then twist it open? (His must not have had anywhere NEAR as much rust as mine had).

Thanks for a great tip. Probably good information for the Restoration Forum too, if you get time.

Frank

*( Voltage regulator ~ funny! )

I was able to get mine apart, like you said.....slightly bending the tabs back and then scrrewing it opne. Yes it is easier said then done and the rust does not help the cause. Sometimes it takes two people to get it up; one to the the booster and other to screw driver n hammer it open (twisting motion). A must so if you have your car apart.....coming fromthe guy who just put his back in without checking

Haha! Don't feel bad ~ I nearly scrapped my entire system before I discovered the Non-Return Valve was installed backwards! That's the little stainless-steel pipe looking thingy mounted on the firewall, hidden under the clamp where the hose from the booster meets the hose from the intake manifold. Once I reversed it (the way it belonged), my brakes worked fine. One end of the valve is so slightly larger than the other, it makes it hard to tell by looking at it which way it should go. It's a one-way vacuum valve, which allows air to flow only one direction (toward the engine).

Thinking about your Post ~ the only way one would know the 'hockey puck' (Reaction Disc) had fallen off would be by unsatisfactory brake performance (after changing everything else in the system) or by shaking the booster drum while listening to it. Know how many times I've done THAT? None.

Clive,

Much thanks for the information and pictures on your brake booster "operation". Looks like your patient made out just fine and your investment in sharing this could could be just what the doctor ordered to solve the brake problem with our system on the 70'. The symptoms sound identical. We'll try this and let you know how it goes.

Thanks again .

Best,

Robin & Colin Foote

_________________
Currently Own - RED 70' 240Z, GREEN 72' 240Z

First Owned - New 72' GREEN 240Z
2nd Owned - Used BLUE 74' 260Z

Clive,

This is very helpful. I am sure that this will save me weeks of time and frustration at some point.

Marc