Rebuilding/Refilling the M/C

81 CSR305

I finally cleaned the old lines and caliper out and refilled them. Unfort. I can't get the M/C to apply pressure. The lines have no air in them, so I believe it's something with the M/C. Where can I get a rebuild kit or how can I test if it's bad? The handle moves just fine but the M/C doesn't seem to move any fluid.

Hi JhnBrackett,
If you have your brake system apart you will have to bleed the air out of the brakes.
The way to do this is to fill the master cylinder with fluid and then pump the brake lever three or four times. While you are still appling pressure on the brake lever, you need to open the bleed screw on the caliper then close it again. The idea is to force air/fluid out of the system. Keep doing this until you get a clear stream of brake fluid coming out of the caliper bleed screw.
If nothing happens after the 3rd or 4th try you will have to do the same thing as above except instead of using the caliper bleed screw, start at the fitting on the master cylinder. Pump the lever 3 or 4 times. then while your still applying pressure to the lever crack loose the hose fitting on the master cylinder. There may or may not be fluid coming out under pressure. Do this 3 or 4 times til you get fluid coming out under pressure.
Once you get fluid moving out of the m/c then you can go back to the caliper bleed screw and continue bleeding there until you get clear fluid streaming out the end. Remember to keep your brake fluid topped up while you are doing this.
Air has a way of getting trapped in the system so if you still have trouble getting them to bleed, you still have air trapped in the system.
If you go to the archives and do a search on bleeding brakes you will find a few good threads on bleeding brakes that get into various ways of doing this.
Hope this helps.

That's not the problem, the air is out of the lines guaranteed. I believe it's a problem with the M/C itself. There's even a possibility the M/C is missing a piece but I can't find an exploded diagram. I bought the bike off someone who had already taken most of it apart a while back. The brake lines & calipers have no air in them & the reservoir is full.

I have been working on cars/trucks/bikes for 40 years, and I have never seen a brake system as hard to bleed as the KZ's

It usually tales about an hour of pumping to get it to finally work. The best way I have found (other than a pressure bleeder) is to put a hose on the bleeder screw, and then stick the hose in a jar setting on the floor with a little brake fluid in it. Fill up the master cylinder, open the bleeder screw, then pump the handle rapidly until the master cylinder is "almost" empty (don't pump it dry), close the bleeder screw and refill the master cylinder, then do it again. After about three times, you should have brakes. There is nothing in the master cylinder but a spring, rubber cup, and a piston, if you have all those, then you have everything.

You have no idea how many times someone has posted here saying "There's no way I have any air left in the system" and then kept bleeding them (usually after arguing) and got the brakes to work.

Post edited by: steell, at: 2005/10/13 19:22

I think the most popular problem is not clearing the TWO holes at the bottom of the master cylinder... most people don't even realize there are two holes. The first is fairly easy to spot - usually it's 1mm or so in diameter. The second hole is a VERY small hole on the output (hose) side of most units - to call it a pin-hole is overstating the size - this is a VERY, VERY small hole - you need to first find it, then clean it out with a very fine wire spike. Unless you do, the master cylinder will not bleed - this is where the air come back up the system and into the body of the master cylinder.

Ironically, you can have a master cylinder work happily with the hole totally blocked - BUT YOU CANNOT BLEED IT after emptying it...

Jeff is onto something here.

I had great difficulty getting my brake pressure up until I reamed out that pinhole. As it is now, I think I overdid the ream job, as my handle pressure is very firm, but it has a longer brake handle travel. (I believe the very small pinhole orifice allows building up some pressure even when the pinhole is uncovered in the begining of the brake handle stroke. Since I made it bigger, I have lost that benefit.)

I suggest you use a very fine wire, and clean out the hole. I used a very small drill bit in my dremil tool, and that was too much.

Have any of you guys used SpeedBleeders. It's a replacement bleeder screw that you open half a turn and just pump and fill, pump and fill till all the air is out then you close it. That way you don't have to open it, pump, hold pressure, close it, repeat process. I just bought some for my Honda but man it makes the whole process effortless. LAter,

I did almost exactly what steell mentioned with the brake system. I can almost guarantee now that it's the 2 holes. I didn't want to drill the back smaller one out cause I wasn't sure if it was supposed to be closed or not. I'll give it a good reaming when I get a chance and see how things go from there.

try bleeding air out m/c first then work your way down to the calipers,if this doesnt do it pull the master cylinder apart and check the o"rings and seals there shouldnt be much else except a spring clean all the parts with alcohal,do not use any oil based products this will destroy the rubber parts.Good luck ,hope this helps .I almost forgot,iallways recomend DOT5 silicone brake fluid after the system has been flushed out.it does not absorb moisture so it will not break down and will not cause rust inside system,and it will not hurt paint even if you spill it directly on it

Post edited by: wireguy, at: 2005/11/18 22:02

If you're having trouble figuring out what to use to prick that hole open, try this. Go over to the toolbox and get a brass bristle wire brush. Then get your pliers and rip one of the bristles out. Put the bristle in a pair of needle-nose vise grips. This tool is handy for both M/C holes and clogged pilot jets. Sometimes I'll use a stainless wire brush, but I always try brass first, as it is softer, and less likely to damage the substrate material.