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blue72

Reaction Disk pictures and walkthrough

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blue72    12

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.

booster3.jpg

 

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.

booster1.jpg

 

 

 

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.

plunger01.jpg

 

 

 

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.

booster2.jpg

 

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.

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blue72    12

From the discussions I read (iirc), it seemed difficult to find a new reaction disk. I re-edited my post to include its dimensions in case anyone needed to make their own.

As a matter of fact, in one of the posts I found on reaction disk problems, calpoly-z mentioned his solution.

I couldn't get it out and did not feel like pulling the booster at the time so i glued a piece of rubber gasket material to the back of the push rod. Brakes have worked great since then.

From this thread: http://forums.hybridz.org/showthread.php?t=134982

 

It is just a simple piece of black rubber, but it can push you near the brink of insanity.

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MatMan    10

This topic is very time appropriate! Two weekends ago, my booster went out on me in my 71. I used an extra 72-74 1/2 booster to swap in.

 

I had to make sure the reaction disk was in it's correct place, and getting the pushrod the correct length was a hassle. Oh, yeah, the clevis pin at the pedal is a different diameter, as well.

 

I have MANY complete 72-74 1/2 brake boosters, and a few others. If anyone needs a reaction disc, or whatever other part, please PM me.

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260DET    25

Good informative post. Had a missing reaction disc problem myself with a 280ZX booster and 1" master cylinder setup, the symptoms are just as blue72 described.

 

New ones are available, in AU anyway, and apparently there are several different ones. The right one for my setup is part # VH2013BK, well it works well so far.

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jerryb    12

Well today I got my reaction disc from Matt Thank you very much!!!!

 

How embarassing...after taking as many old Zs apart as I have...I can honestly say I have never seen one of these before. I always though the reaction disc was that seal ring! It looks like a round piece of silicone! Thats it!

 

I always had a long peddle stroke...learned to live with it and played many days at the track out braking most others. So we continue to learn.

 

Makes me wonder....how difficult could it be to remanufacture these. Small flat disc made of silicone. I will have to look into it! Wonder how it got its name?

 

Do I just glue it onto the piston?

 

Again...thanx greatly Mat!!!!! You represent the forum well!!!

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blue72    12

All I used was two part epoxy to glue mine onto the back of the push rod. Just some Permatex 4 minute multi-metal stuff I had laying around. It was stuck on there for good after three minutes. I did have to wipe off the excess where it squeezed out, before it hardened.

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Guest msthaiii   
Guest msthaiii

so from my understanding, a new brake booster should work also? but i should try this first before spending money?

 

also, whenever i step on the brake, i hear something like an air hiss.. is that a sign that i should get a new one or is that normal? that is for a stock 260z brake booster [at least i think so]..

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brokebolt    1
so from my understanding, a new brake booster should work also? but i should try this first before spending money?

 

also, whenever i step on the brake, i hear something like an air hiss.. is that a sign that i should get a new one or is that normal? that is for a stock 260z brake booster [at least i think so]..

 

The hiss is a vacuum air leak. Some times the engine RPM's will raise slightly do to the addition of air into the intake. I would replace it as you "could" cause engine damage, and your brake booster is a vacuum device so you are loosing brake pedal efficiency, meaning you have to push the pedal harder to make the same fluid pressure in the brake lines. (Some older cars, race cars, and aftermarket set ups do not use a brake booster.)

 

Follow his links posted here for the set up on the master cylinder push rod length. They may need to be adjusted for the 280ZX master cylinder upgrade. I say this as when I did a 280ZX upgrade and Toyota 4X4 calipers my rod was in no need of adjustment.

 

blue72, thanks for the pics.

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barbom143    10

never heard of a reaction disk until today. I took my booster off and the metal part of the plunger came out but I couldn't find the disk in the housing. looking into where the metal plunger fits, I see "rubber" not medal. Is it possible that the disk is stuck in the indention and not to the plunger?

 

I have another post Help! No Brakes with a full description of my problem. Any hep woul appreciated.

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LoneStarS30Z    11

Lucky. I could not for the life of me get anywhere close to it, much less see it. I would spot it for a split second then lose it again.

 

I hate to admit it, but eventually I said to hell with it. I epoxied a 2 quarters together with a nickel in between them and attached them to the plunger and called it a day lol.

 

I will take another whack at it this weekend if the car doesn't need anymore immediate work.

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chunk    10

Since my 78 280z is basically the oldest vehicle I have ever driven, I never thought much of the crappy brakes. Plus I only drove the car a few times before it was gutted for a JTR style chevy conversion.

 

While cleaning up the brake booster I heard the disc rattling around inside. I shook it out instead of trying to open up the brake booster. It didn't take long for that thing to fall out. I roughed up the metal and the rubber a little bit and super glued the pieces together.

 

Good to know the brakes wont be so crappy after adding a few horses.

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