OMG Dr., nothing like that level of effort! You have a fundamental issue there, and I think what I'd do is to disconnect the metal line and remove the master cyl, and use compressed air to blow-through both ends, trying to get things removed. Of course, you need to blow
into some type of containment vessel. Something like a flexible line emptying into a 5 gallon bucket, where the air & whatever's obstructing things can fly-out without going everywhere, ruining your paint, and contaminating your room.
If I had to guess, I'd say that you have a partial failure of the hose inner lining where some of the debris is occasionally obstructing that small, forward pinhole in the floor of the master cyl. Removal of the master cyl from the handlebar, and flushing it out may help removal of anything in the area of the piston, but distal from there, towards the caliper (if you were doing the brakes) or in this case, the slave cyl, you need to disconnect the slave cyl and clear-out the rigid line. I suggest also removing the slave cyl (requires an engine side case cover gasket) to flush it out as-well. Flushing fluids into something which would allow you to see what's removed/drained, wouold show you the potential pieces of what's causing your obstruction.
I'd venture to say that if the rule of replacement is 'every two years,' 99% of us are guilty as-charged.
I have one for you. I bought an old 1974 SOHC Honda 750 from my machinist. It sat unused for years, and in a fit of cleaning up/cleaning out, I decided to move it. I like to be able to
stop things that heavy which I put in-motion, and that required disassembly of the caliper and the master cyl. Both put up a fuss to come-apart. One bolt for the caliper halves I had to drill-out repeatedly in different sizes, until the bolt finally was able to come-out. A drill press made this relatively painless, including the use of good drill bits.
The second thing was the master cyl had a frozen piston. On a VMax, where the threaded hose female master cyl port comes in at a 90 degree angle, it's harder to do anything to remove the piston, maybe by drilling out the piston shaft from the lever end, or heating up the body and quenching it with PB Blaster in the piston chamber. However, on a SOHC '69-'78 Honda disc brake, the female threaded master cyl port is directly-in-line with the piston.
I thought about what to do, and came-up with a simple, inexpensive tool to drive-out a frozen piston, without having to use heat. Here it is.
You see it's a couple pieces of PVC, and the largest solid steel rod which fits into the female threaded end of the master cyl. I used a NPT male plug to give me a better chance of hitting the rod, it's drilled halfway-through the plug length to the diameter/size of the rod. It works well, a couple good whacks and the rod popped-out.
Here is a pic of seeing if my piston removal, bore clean-up and piston replacement was going to work:
I used a 1/4" long ratchet extension with something like 220 grit sandpaper wrapped around the ratchet extension , in a cordless drill, to quickly hone out the master cyl. Probably 2 minutes and it was done, checking several times to ensure I wasn't removing too-much from the bore.
That left the hose. It was to OEM rubber hose, not this SS hose pictured here. And like what Dr.Doom may be facing, it was obstructed. Except in my case, the hose appeared to be solidly-blocked. I used a red want & brake cleaner into the end of the hose which seemed open for the longer distance, and the brake cleaner wanted to spurt everywhere after a short spritz. I recommend using a heavy rag or a bunch of paper towels over the end where you're using the red wand. You need to stop the brake fluid from getting everywhere, including your eyes! Also, don't just throw the solvent-laden towels or rags into your trashcan, as you could have a case of spontaneous combustion, and start a fire. Use either a rated waste container w/a spring-loaded lid, or store the soaked rags.towels outside and away from anything flammable.
The next thing I used was a length of spring steel hobby wire smaller than the hose diameter, and I fed the wire into the hose until I hit the obstruction. Using
a bit of the old in/out to quote Alex the Droog from Burgess's
A Clockwork Orange, the hose was soon opened. I used the red wand and brake cleaner until the liquid was running clean. Then I flushed it with a bit of fresh brake fluid, and assembled things as you see to test the work. No leaks, the caliper worked and retracted properly, and what had been an inoperable system, was returned to service, as a functional disc brake suitable at-least for stopping a roll-around bike.
There is another part to this about using a lever-action grease gun to dislodge the one moving piston of a SOHC Honda fixed piston/moving piston brake set-up. Open the bleeder valve, and inject the caliper until the piston pops-out. Many people use an air compressor to remove piston(s) but they only generate up-to about 160 psi, while a lever-piston grease gun can approach 2,000 psi. I've never lost to a stuck piston using the lever-action grease gun, once the compressed air doesn't pop-out the piston.
I hope this helps.
