1981 GPz 550 Rear Master Cylinder and Front Caliper.

Hi all,

Finally got around to replacing the break fluid on my rear and front brakes after they were refurbished. Long story short, I have a leak on what looks like the piston cup of the front right caliper, as well as brake fluid leaking out of the rear master cylinder at the push rod end. I was "told" by the guy that did the job that after he was completed with the refurb that everything was leak free...as he had tested it. All seals were also good apparently. Since the entire brake system was stored dry for about 4 months, is there a slight possibility that the seals only need to be in contact with brake fluid for a while in order to "swell" so that the seals will seal properly? It might be a long shot, but worth asking.

Thanks!

Storage shouldn't cause leaks.

What was done to the system? If the master cylinder has corrosion pitting in the bore that's bad (obviously). Same thing for the caliper pistons. While restoring the brake system on my '81 750 only one of four pistons wasn't pitted, and both masters were too. I had to replace all those parts. Of course you have to break down the calipers and masters to properly inspect them so hopefully that's been done on your bike already. The stock rubber brake lines should be replaced while you are at it.

Grab your wallet and a box of tissues.

One or two ply tissues Nessism?

as far as the rear master cylinder, I had the same problem after rebuild and was told to put a brick or something heavy enough to keep the brake pedal down overnight, next day I cleaned off whatever fluid leaked out ....& no more leak ...It's worth a try
Pete

Forget trying to get a rebuild kit for the rear master. Even if you could, the cylinder is probably pitted. No worries though... you can get a good, mid-2000's Ninja 250 master to swap in. I think I paid around $10 for one 5 years ago. You just have to fab some spacers and a pushrod adapter. Not too tough if you have access to some basic power tools and a good bench/vise.

For the pushrod, you can probably just use a threaded rod, M8 x 1.25. See link for details.

www.kzrider.com/forum/5-chassis/531950-a...l=component&type=raw

Thanks Pete and Loud. Going to try having the brake pedal applied overnight first, if that fails I will go the Ninja route. Now just to try and sort out the front caliper.

So I found the culprit. Any possibility of honing this out? Seals still looked to be in really good condition at least, but I am sure any level of honing might cause possible bigger leaks, or am I wrong?

That pitting at the lip is not the problem. It's the pitting further in that is the problem.

Roger.

That master looks like junk. You can clearly see all kinds of surface imperfections in the bore. Masters like this should not be honed. Get a new one.

That sort of corrosion damage will be throughout the system. Hope you broke down the calipers and cleaned out all the gunk. Replace the seals and brake lines, and any pistons that have pitting.

I have the exact same bike. My rear looked like that when I finally gave up on it, but the rear caliper was fine. The front master was bad too, but I was able to get a decent used one for the front. The front calipers were ok, not great, but ok, and worked.

Some feedback. There is a place called SA Master Sleeve about 30 minutes drive from me that came highly recommended by a BMW Motorrad mechanic. Decided to take the rear master and front calipers there. For the equivalent of $130 I got a completelty resleeved master with a new piston and seals, as well as two new caliper pistons and seals. And to top it all off the master sleeve and pistons come with a lifetime guarentee and the seals with 3 months. And guess what...zero leaks. One more step in the right direction.