1979 KZ650 SR rear master cylinder woes

I've been working on a restoration since last fall and the last thing left to get sorted is the rear master cylinder. I left it till last because I thought it would be the easiest fix. I've pulled off the old one which appears to be fine and thoroughly cleaned everything up but no matter how long I try to bleed I can't get any pressure whatsoever to the caliper. I had the same problem with the front but it was easy to find an aftermarket front master cylinder and that worked great. I can't find any rebuild parts and I'm still not 100% convinced that that's even the problem. Even if the seal and cup are worn, shouldn't I be getting at least some resistance ?? Any help or advice would be greatly appreciated.
Gary

Try back flushing. I connect a 60cc syringe filled with brake fluid to the caliber bleed fitting and push fluid into the master.

Do you have a brake bleeding apparatus connected? usually clear tubing with one end submersed in a clear jar of brake fluid. and it helps if you prime the tubing with brake fluid beforehand. Or if not already installed, I would install a speed bleeder >> makes bleeding the brakes much easier.

thanks very much for the reply. I'll give that a try

I do not at this point. I guess I convinced myself something is wrong with the master because when I disconnect the brake line from the master and pump the brake still no fluid comes out. Shouldn't fluid be pouring out of the master cylinder with the brake line disconnected ?
Thanks very much for the reply.

Not poring out, but you should get a squirt each time you press the piston in. The piston is spring loaded. Does it pop back out after pressed in?

ckahleer wrote: Not poring out, but you should get a squirt each time you press the piston in. The piston is spring loaded. Does it pop back out after pressed in?

Yes it does. I had the whole thing apart to clean it all up and sprayed it down with brake cleaner. All the parts look good but I understand you can't judge the condition of the seals by looks. When I compressed the piston in order to reattach the circlip it all seems to be working freely popping right back up after compression.
I would assume that I would get at least a squirt each time I depress the peddle but nothing, even with the brake line removed. There are no fluid leaks anywhere so it really has me puzzled. But as I mentioned, the front brake was doing the exact same thing until I put on a new aftermarket master cylinder. It took just a few minutes to bleed that system and it now works great. Could the seals be that worn out that they are building up absolutely no pressure ? Seems unlikely to me, especially because it's not leaking. I did have some brake before I took everything apart. Very little but enough that I couldn't turn the rear wheel when up on the centerstand with rear brake applied. Now nothing at all.

Short answer is yes and no, you should get some fluid out of the master cylinder with the line off unless it is air locked. Are you positive your brake pedal is releasing all the way? It must return to a position that opens the passages in the master cylinder. If the adjustment bolt for the brake pedal isn't adjusted right the pedal won't release all the way which in turn doesn't allow the passages in the master cylinder to open up and allow fluid to pass into the cylinder or back to the reservoir. Having said all that, have you replaced anything in the master cylinder such as seals? If not now is the time to do so. You should take a real good look at the interior of the master cylinder where the piston travels and make sure it is spotless. Any corrosion in there can really mess things up for you. There are a couple of different ways of bleeding a brake system but they all accomplish the same thing hopefully. If you use the old fashioned way you install a clear tight fitting hose on the bleeder nipple and put the free end in a supply of new brake fluid. Pump up the pedal 5 times or so and hold it down while opening the bleeder. You should get fluid out along with air that you will see as bubbles in the tube. Keep the free end of the tube immersed in the brake fluid while doing this! Tighten the bleeder valve before releasing the pedal and repeat. It may take many, many repetitions of doing this but eventually you should develop some resistance in the pedal. Keep an eye on the air bubbles coming out of the bleeder valve. Once all the air is removed from the master cylinder and caliper you should develop nice firm pedal resistance. If it is spongey feeling you still have air in the system or some other problem. Even though it is about as basic a brake system as you could ask for it will and can give you fits at times. Patience is the word of the day when bleeding brakes. Lastly make sure to keep an eye on the brake fluid level in the reservoir while you are bleeding it. Nothing is worse than bleeding your system and making some headway only to run low on brake fluid in the reservoir and having to start all over. Remember your brakes and tires are absolutely critical to your well being! I am not throwing rocks, but if you have any doubt(s) about what you are doing see if you can get some help from someone with more experience. Good Luck!
Rick H.

I had a similar problem with my first MC rebuild. I assembled it dry. It wouldn’t work. Then I used grease to assemble, and things worked but I got concerned that I may have contaminated the system with the dissimilar media. So I Remove the entire rear brake system. Replaced the hose, disassembled both the caliper and MC and throughly cleaned. This time I assembled the parts with brake friendly “Red Rubber Grease”. A little extra pumping with the bleeder loose and a clear tube in a jar full of clean fluid. I kept the open end of the hose submerged in fluid and the cap on the MC tight. Checked the MC fluid every second pump and shortly it started to build pressure. Then I bled the system normally, everything closed, pumping the master cylinder several times, holding pressure, and releasing the bleeder screw, then tightening the bleeder screw before releasing the pedal.
If you had low pressure before cleaning and rebuild, I’d replace the seals and piston. You need to run a ball hone in the bore to break the glaze and give the new seal a surface to bite into and work. MOST MC bores are either 1/2” or 5/8. Ball hone are easy to find. Buy fine, medium, and corse grit. I have had success with a 320 grit as the final cut. I’ve rebuilt several and have one going on 5 season and every spring so far the rear brake is firm on the first pump.

Here’s a previous post on the same subject
www.kzrider.com/forum/5-chassis/466301-r...er-cylinder-79-kz650

No eBay APP ID and/or Cert ID defined in Kunena configurationHere’s an exploded view so you can search for individual parts.
www.partzilla.com/catalog/kawasaki/motor...cylinder-brake-pedal

Just a quick Fleabay search showed the primary cup is available

Thanks everyone for all the input. It's really great that folks take the time to respond here and I do appreciate it. I'm 64 years old and have been working on vehicles, cars and bikes for quite some time and while I'm nowhere near an expert I do have general knowledge of brake bleeding and have done it numerous times. That's why this problem is not making any sense to me. I would love to replace the cup and the seal but extensive searching shows the parts are not available. If you look at the partzilla diagram, which i found days ago, it shows "unavailable" for everything I need. I've searched Ebay for days and came up with nothing. Z1 Enterprises, Dennis Kirk. The folks at my local vintage repair shop who are very good also tried for me and came up empty. If I'm not mistaken the KZ1000 master cylinder has a larger bore, 5/8, so I don't believe that cup will fit the KZ650. I may be wrong.
I've also been searching these forums for days before posting for help. Someone said the KZ1000 master cylinder will bolt directly to my bike. Someone also said a master cylinder from a 2006 Vstrom will work on a KZ1000. So that would mean the one from the Vstrom would also fit the 650. I have no idea if this is true but I ordered one off Ebay and I'll see if I can rig it up. I'll keep you posted. Thanks very much for the advice.
Gary