Master Cylinder or Brake Lines

So I know there are a lot of master cylinder questions on here and I don't want to repeat anything but I have gotten mixed answers and figured I would come here where the KZ knowledge is for some clarification.
I have new stainless steel brake lines for my '81 CSR 1000 but I haven't installed them yet and I am wondering whether or not I should also do the master cylinder at the same time. It started with my brakes being soft in the heat so I assumed, and was told, it was my old lines deteriorating even though they appear to be in pretty good visible shape and do not leak (they do have very tiny cracking and look a little weathered). From what I can see looking through the window in the MC the brake fluid is also older looking but doesn't appear to be full of moisture and is at an acceptable level. However, even today when it was only about 35 degrees F (usually a temp where the brakes are a little harder), if I kept pulling on the brake handle it would go all the way to the bar and never hit a point where it got really hard or too hard to pull further. I know it could probably use a fluid change and I'm sure the old rubber lines have some give but my thought now is that the master cylinder is also bad since the brake handle can always be pulled to the bar no matter how many times they are pumped. The calipers are in good shape and there are no leaks of any kind.
So, what do you think? Should I also do the master cylinder while I'm at it? I would hate to do new lines, pads, fluid and get the system bled only to find out my MC is shot or will be soon. Judging by how weathered it is, it must be the original and probably hasn't ever been touched. Thanks for any help or suggestions.

I believe in changing the brake fluids at least one a year, as most are Hydroscopic(absorbs water)This water is what corrodes the pistons in the brake systems. The fluid may look good, but unless it's tested for moisture content, I'd say replace it with fresh fluid.

The stock brake lines will expand when pressure is applied, so you're changing to the SS braided lines is a good step.

I changed my master to a more modern one off of an 01 sv650 also with stainless lines , and wow my breaks are better on my kz than on my 07 gsxr. Just my .02 .

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Figured I would just update my old thread. I put on new pads, SS lines, and new fluid. I initially bled the brakes then drove the bike for a while and bled them again so there is no air left in the system. However, I still have a lot a play in my brakes and I can still pull the brake handle to the bar so I'm guessing my master cylinder is getting a little weak. At this point I have several options and not sure which route to go. I could rebuild my old master cylinder but I'm not sure how effective this would be and I'm not sure whether or not the bore is also worn out. My other option is replacing it. An OEM replacement is expensive though I have found some universal motorcycle master cylinders that have the right fitting and mount but there are only like 12.7mm-13mm bore. Stock bore is 5/8 inch so I know 13mm is smaller but just wondering if it will still work despite being a little smaller. Otherwise, does anyone know of any master cylinders off newer bikes that would be a good replacement? I know the one off an sv650 will work (as stated earlier in this thread) but does anyone know of any others, would just like some options. Thanks again for any help.

I dont think its your MC, its probly air still trapped in the system.

Anywho... clean up your MC anyways. Just pull all the guts on it and wash/clean them up like new.
Then hone the inside of the MC with some 0000 (4 zero) steel wool for several minutes, then wash and rinse it out repeatedly till shiney and new like.

Lubricate and reasemble things, and I guarantee this will make a big improvement in the operation and longevity of your MC - It just needs some attention!

You may need a rebuild kit if the diaphrams are damaged, but not likely. Too much $$$

cpg wrote: ... new pads, SS lines, and new fluid. I initially bled the brakes then drove the bike for a while and bled them again so there is no air left in the system. However, I still have a lot a play in my brakes and I can still pull the brake handle to the bar so I'm guessing my master cylinder is getting a little weak. At this point I have several options and not sure which route to go. I could rebuild my old master cylinder but I'm not sure how effective this would be and I'm not sure whether or not the bore is also worn out....

Where the master cylinder isn't leaking, it's likely not getting a little weak, and the bore is not worn out .

Most probably some air remains in the system. Although it may be invisible while bleeding.

Some methods of filling and bleeding are more efficient than others at preventing introduction of air upon filling and later purging air from the system.

And clean fresh fluid should be used -- undisturbed, not roiled in any manner before using, and definitely never dropped or shaken.

Good Fortune!

Where the m/c is pushing enough fluid to bleed in the normal manner by using the bleed valve, am thinking the m/c supply and relief ports are probably clear and okay.

Good Fortune!

Thanks guys. Guess I will take it apart to make sure it is all nice and clean and then bleed the brakes a couple times.

cpg wrote: ... will take it apart to make sure it is all nice and clean....

Wouldn't expect dis-assembly of m/c to accomplish very much where it isn't leaking and each stroke is pushing the proper volume of fluid.

Good Fortune!

You will have some chemically induced corrosion on the wall of the bore but no actual friction induced wear as such because the only thing in contact with the bore are the primary cup and the other one(cant recall the correct name) which are both rubber.
As TeK says,restore the bore and generally service it.My rear mc played up and I read the same advice here which I followed, I now have rock hard brakes.
Also, the primary cup and its friend are available,the 1000 and 500 share a number of part numbers.If you need them I'll dig out the suppliers.
I recall Zpower in the UK have them.

Yeah I found a rebuild kit for like $35.00 that includes all new seals. I will probably clean the bore as recommended and rebuild it while I have it apart. Once I drained the brakes I found the old fluid was worse than I originally thought and that it was pretty dirty. I tried to clean it up as good as possible but taking it apart and doing a full cleaning is probably the best way to get all the old gunk out and new seals should ensure the brakes stay hard. Guessing all the old gunk is the root of my problem but for about $30 I might as well freshen it up while I have it apart.