Believe it or not, my brakes are too good! What do I do?

[Guest jt240z]

I know this sounds strange, but here it goes. First off, my car is a 1972 240Z with a L28ET turbo transplant. I already had the following mods on my brakes:

 

Front:

1984 300ZX vented front rotors.

1990 Toyota 4 Runner 4 piston calipers.

1982 280ZX master cylinder

 

Rear:

1979 280ZX Rear disk calipers

Maxima rotors

 

My master vac went out a couple of days ago, so I decided to do another upgrade while I was at it. I replaced the stock 6" 72 brake booster with a rebuilt 7.5" one from a 1975 280Z. I had to drill 4 new holes in the fire wall to mount it, but everything went in just fine. Since I now had a 280 brake system, I adjusted the rod for 9.8mm extension past the aluminum spacer like the 280Z and ZX factory manuals say to. The 240 manual says to adjust it to 4mm past the spacer.

 

Now my brakes have always been a little weak, so I was expecting some improvement, but nothing like what I have now. My brakes are so sensitive that all I have to do is lightly press on the bake pedal and the car hauls to a stop. The wheels don't pull or lock up (I can lock them if I press hard enough), but the car feels like it has power brakes from a late 60's early 70's GM car. It seems like I went from what felt like weak brake fade before the booster change(while the booster was still working) to supper hyper sensitive bakes now. I own 2 other Z's and neither one has brakes this sensitive. I checked the service manuals, but none of them have a trouble shooting section on brakes that are too good.

 

What can make the brakes so hyper sensitive? What can I do to reduce this? Does the rod length on the brake booster effect how sensitive the brakes are? Could the new booster be defective?

 

I guess this is a case of "too much of a good thing".

[David K]

I had the same problem when i replaced my booster. When i went around the corner, it STOPPED.....and i mean STOPPED. Too much for me, and it was dragging kinda. You need to go under there, take out the pin that holds the pedal to the booster rod, and adjust the rod by backing it off a little.

 

good luck,

 

cheers,

 

dk

[Guest jt240z]

Do you screw it in or out to reduce this sensitivity?

[David K]

Do you screw it in or out to reduce this sensitivity?

 

It goes in. The longer the rod length, the sooner the brakes kick in. So if it is out more, the pedal will need to be pressed less to create pressure in the system.

[260DET]

You sure that adjusting the rod will cure excessive sensitivity? When my larger than stock booster was rebuilt it was suggested by the rebuilder that it be modified internally to reduce the initial sudden bite. Which was done with great results.

 

Don't know what he did but it was simple, apparently.

[fl327]

brakes are broken, ill trade you for a stock setup so you can drive again

do it.l

[Guest jt240z]

I did some more searching on other web sites and I've come to the conclusion that my rebuilt booster is bad. Some have suggested that if I adjusted the length of the push rod after the unit was installed, the reaction disc may have fallen out. This would cause the symptoms that I've described. I checked through 3 different year factory service manuals and found in the 1971 manual this warning:

 

Note: When adjusting the depth, face the push rod end upward so that the reaction disc is not dropped off into the master vac.

 

I wish the rebuild installation instructions had mentioned this. I did notice that the adjustment rod was taped down so that it would not move during shipment. Well I ordered a replacement booster and that one should arrive today. I'll let you know what happens after I replace it.

 

This all does lead me to the conclusion that a booster could be modified to provide a larger pedal to fluid drive ratio. This would give more braking force for the same amount of pedal pressure. The desired result would be a steeper slope of pedal pressure v/s drive fluid but will also have less pedal feedback. Hopefully not as little feedback as what I just experienced. I may spend some time looking into the workings of the boosters used in the 240 and 280Z's to see what could be done. If I come to any conclusions, I'll let you know.

[Guest jt240z]

Turned out to be the booster. Looks like the reaction disc was missing or fell out of position during assembly.