over-sensitive rear disk, KZ750 twin

I'm thinking about buying a 1978 KZ750B. I'm happy with the bike, and if it were to be my only ride I would probably buy it. However, the rear brake is vastly more sensitive than other bikes I've ridden with drum rears--I accidentally locked it up on a relatively mild stop on the test ride. This makes me nervous, since it isn't going to be my only ride--I'm afraid that in an emergency I'll forget which bike I'm on and lock up at a higher speed.

The tires would be replaced pretty much as soon as I got it home--It has Cheng Shins, seller says that they were on it when he got it, and are pretty old. Do tires make *that* much difference? If not, is ther anything that can be done to make the rear match my other bikes?

I'd remove the rear wheel, inspecting the drum and brake shoes for wear. Some of the rear drumed Kawasaki's are experiancing adhesive/glue failure holding the brake shoes together. While you're in there, clean and lube the brake mechanism.

As for the tires;most motorcycle tires should not be used if they are more than 5 years old, due to loss of flexibility when the chemicals that make up the tire leave, due to Ozone and Sunlight.This loss makes the tire(s)stiff and they do not become responsive to road conditions.

There should be a date code molded/impressed in the tire sidewalls about pressure too.

Wasn't quite clear. The Kawasaki has a disk rear, it is my current bikes that have drums. I'm not going to bother looking at the date codes on the current tires, the Cheng Shin is enough to convicne me to change them.


The glue failure isn't limited to Kawasaki, the standard greeting to a new member on the Yamaha XJ forum is basically "Welcome--check your rear shoes for delamination"

So, your potential new bike is the one with an overly sensitive rear brake? I'd change the brake fluid, making sure to use either DOT 3 or DOT 4 brake fluid. The DOT 5 is a Silicone based fluid, not compatable with the first two mentioned. Most brake fluid is Hydroscopic, meaning it will absorb moisture, slowly contaminating the fluid.

Brake system contamination
The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some car owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. Older brake systems should be flushed only with DOT 3 or 4.

If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.

Brake pads can get ultra grabby if they are contaminated with oil or grease. I ride a 79 B4 and the rear brake is not overly sensitive at all. Yours needs work. I would replace the pads just for smart money, they don't cost much. While the wheel is off clean the glaze off the disc face with some 120 paper and a sanding block.

I agree to flushing all the brake fluid from the cylinder to the wheel, replace with ew fluid and bleed. problem should be gone/

I ride a '79 750 and I agree with Sevesteen here. I think the rear brake is overly sensitive by design. It's a great big single disc and if you're not used to it, it is very easy to lock the rear.

I don't think new tires will make much difference. I think this is a design flaw with this bike. I've gotten used to it and it doesn't bother me. The rear brake just takes a light touch.

If you think you're going to have a problem adjusting to that, then this may be a real problem for you.

I've frequently wondered whether it would be possible to swap a smaller disc, caliper and master cylinder from another KZ onto the bike. But like I said, I just got used to mine.