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Hi all,

I recently purchased a '78 kz400 B1, and the front brake caliper will seize when applied until the bleed valve is opened. I am thinking that I will need to replace the caliper or at least rebuild it. What years and models have calipers that will interchange? I am new to these bikes and any input would be greatly appreciated.

If bleeding the fluid releases the caliper, then the problem would more likely be the master cylinder.

Thanks, I'll check it out then, I haven't started digging in to it yet.

Calipers and master cylinders can be dismantled and reassembled without issue, if they're in good nick you won't need any new seals or gaskets. If there's nothing wrong with a particular part, you can pull it apart, inspect, and reassemble. It's worth doing if you're unsure of your brakes.

Take your caliper off the disc, remove the pads, and then pump the brake lever until the cylinder pops out. Wear gloves and hold everything over a container, because it gets very messy! You can then remove the boot, cylinder, and seal, and inspect the caliper. They're really quite simple, and any issues will be obvious (usually damaged seals or rusted cylinders). It's easier to do the caliper first as it's easier to remove the cylinder by pumping the brake lever.

If that's fine (or even if it's not), you can dismantle the master cylinder by removing it from the handlebars, removing the lever ans then removing the boot and the clips holding the plunder in place, and pulling out the plunger and seals. Again, inspect and replace anything that's worn, and reassemble.

If a caliper is sized like yours the the rubber gasket of the caliper piston in the caliper bore is leaking and cause a blow by off the break liquid.
This create a hydro lock that won´t let the brake liquid flow back to the master cylinder reservoir .