I have a rear brake issue I don't understand. The system bleeds ok and I have pressure on the pedal and the brakes work, but hold that pedal down for a while and it sinks to the end of travel and the brakes fail. No good at all at traffic lights on a hill

I thought the caliper was leaking and put a seal kit into it today as well as a new stainless hose, the original had cracks appearing, and the issue is still there. I don't seem to be loosing any fluid either, if anything, it sort of looks like the level in the reservoir goes up a little as the pedal goes to full travel and I can't find any leaks anywhere.

The only thing left is the rear master but they don't make a kit for it so a new one will be around $130 delivered and I figured I'd try pulling the master apart for a look see before ordering a new one.
 
I just wanted to check that there isn't something else to look at before spending money .

commonly the bores become pitted (water contamination ?)

I have successfully pulled the piston and cleaned up the bore with a makeshift hone (was on the front master ...and a few others )

get some 10 mm dowel and cut a cross in the end ...deep enough to hold some wet and dry  400 or higher grit ....of about 15mm in width and maybe 40-50 mm long ,,wrap the wet and dry around the dowel in a layered fashion with one end through the cut in the dowel

then progressively work the cylinder with the hone with a cordless drill ...easy does it !

a new piston seal would probably be best option if they were available because the "edge " disappears real quick when travelling along a pitted bore


I have got some from some smaller carparts dealers before  mostly pbr because they were pretty common ...but worth asking around

 
 
looks as though you have nothing to loose but for the trying huh ?

Cheers Charlie, yes it's definitely the master cylinder. I pulled it off the bike and the push rod and boot comes away from the piston easily and when I drive the piston in/out I see fluid seeping back out around it. 

The piston is held in there with a circlip which I can't get out. I've managed to bend it a little but it don't want to come out. I might see if I can get/invent a circlip tool before I destroy it. The one I have works in the wrong direction.

real quick run to supercheap perhaps and get the right ones before you destroy the whole thing

Can't even get onto their website, imagine the crowds at the shop today  

Found some tiny pliers and out she came. There is a plastic tube at the bottom below the bore with an o'ring on top of it. I think that one is leaking a bit as well. Looks a lot like a 1/2" one off the garden hoses. I'll try a cleanup job on the bore and look around for another o'ring.  Cheers.

If the pressure falls and no fluid is lost, it must be that the pressure loss is due to fluid passing the piston. If the O-ring fails, as its purpose is to keep fluid inside the chamber, I'd expect to see a leak.

Damage to the piston bore is possible but much more likely is the rubber seal on the piston itself allowing fluid to seep past while the piston is depressed for any length of time.

Yep, the piston seal is between the piston and the spring in that photo. Fluid is passing that, being stopped by the 1/2" o'ring and going up the tube to reservoir, hence no outside leaks. I couldn't fix it, ordered a new one.

My K100Lt has been sitting in the garage for a couple of months, and I noticed the rear brake lever travel had increased by about a inch and half before activating the brake.

The fluid level was normal and I couldn't find any leaks, so I assumed air must be in the system.

I purchased a one way valve little bleeder kit and bottle of dot 4 fluid and started to bleed the system.

At first a small amount of fluid appeared in the tube about an inch long and then some air bubbles appeared, as I continued to pump the pedal nothing else came out and the reservoir level stayed the same.

Now I have no brake activation at all.

Could this be a master cylinder fault.

sounds to me like the master cylinder piston has stuck at the bottom of the bore.  Replacing the seals and honing the MC is a good place to start and it will probably finish there as well. Tip there is a small socket head cap screw (allen head) on the back of the MC which needs to be removed to allow the piston to come out.

When I replaced my master cylinder before xmas, it did the same thing when I went to bleed it. No brakes at all. I tried pumping fluid in from the calipers but in the end I had to pour some fluid in the top of the master cylinder where the brake line attaches. I guess that primed the line

Thanks for the information, will inspect closer.