Well, either way, you're going to remove any residual crud.
Using a bleeder tool like a Mityvac usually does a good job of removing things, but if you have some-sort of hardened crud in the line (I see you say it works fine now) then that's best to be hose replacement time. I'm fortunate that the guy whose shop I frequent makes stainless-steel hoses any way I need. The hose line on the SOHC 1974 Honda 750 was not flowing when I disassembled the entire front brake system, I used some brake cleaner thru a red plastic tube which comes w/aerosol products like that, and was able to get out the dried brake fluid which had created the obstruction. I bled the syatem and set it up, off the bike, and checked the function. It was working, the caliper was moving/retracting as it should. I was interested to see if the hose line was bulging while under-pressure, it wasn't. For rolling the bike around, (static use) it works fine, but before I use that caliper and MC on a bike under its own power, the hose will be replaced.
For your situation, here is what I'd do: remove the side cover behind which the slave cyl resides. Use the proper allen key in a 3/8" drive socket & extension, and remove the slave cyl. An obvious sign of a leaking slave cyl is an accumulation of fluid beneath the bike, and in the area of inside of the left operator's footpeg. That, and a failure of the clutch to work. Remove the slave cyl, and disassemble it, and inspect the O-ring & dust seal. As you've come this far, if the bore appears to be good, maybe just use some 120/220 sandpaper to remove anything from the cyl bore, and also clean-up the piston.
You can use a vinyl vacuum cap to plug-off the end of the clutch line, or the proper size of rubber tubing over the clutch line & a machine screw in the opposite, open end of the tubing. Remember that brake fluid we commonly use (DOT 3, DOT 4) will remove paint quickly. I use aerosol brake cleaner to cleanse any spilled brake fluid onto the bike's painted surfaces.
Attach a length of clear plastic tubing to the clutch rigid line on the bike, and drain it into a cup to see what comes out, filling the MC once or twice, to continue to flush-out anything in the line. Once you have done that, and you're satisfied that nothing in the way of contaminants is still in the line, you can throw a rebuild kit onto the MC, and reassemble the slave cyl (rebuild kits are available from Yamaha and aftermarket sources, if you go that route, get one from a recognized supplier like K & L), re-install it, and fill the system. If you choose to do a reverse-bleed to fill the system, you will be pushing upwards the brake fluid, and then you will see it gradually fill the MC.
If to fill the brake or clutch system, you do it the traditional way, before I install a clutch master cyl or a brake caliper, I fill the slave cyl or caliper to capacity, and then install it/them. Now you're just dumping brake fluid into the master cyl, and you need to go thru the:
- fill MC with brake fluid
- open bleeder at slave cyl/caliper
- fully-squeeze and hold-in the lever to the handlebar
- close the bleeder valve
- release the brake/clutch lever held to the handlebar
- repeat as many times as needed to see a pull of the lever result in a solid column of brake fluid enter the clear plastic hose attached to the bleeder valve
Now that you have no air entering the line attached to the bleeder valve, performing several rapid pumpings of the brake/clutch lever should get you a firm resistance. If it doesn't, you need to bleed more. Of course the clutch lever should come-back to the bar, while the brake lever should stop about 2/3 of the way to the handlebar.
That should put you back in business, safely. I do not recommend the reverse-bleed on any ABS vehicle.
You said you wire-wheeled the MC, did you do it on a benchtop wire wheel, or did you do it in a drill attachment? or a Dremel? The Dremel is a great tool, I use mine frequently, and while there are more-robust rotary tools
Rotary Tools | Foredom Electric Company for considerably more $$, I find the Dremel works for me. I use the 3 ft flexible wire extension w/mine, and that does a good job. Note that laying it out so the flexible line is as-straight as-possible results in the highest RPM from the motor, while having the cable in an S, or a U shape, slows-down the speed of the tool.
A word of caution, anything which gives you pause about returning it to service,
always err on the side of caution, and retire/replace it. You need the MC's to function faultlessly. If, after your best efforts, they don't, then replace the faulty component(s). We are fortunate that many of the VMax Gen 1 parts are still available. I generally just replace a MC rather than rebuild it, if the vehicle has them available. You can also search online for replacements, the bore for both the clutch cyl and the front brake cyl is 5/8 inch. Anything you buy should be either 5/8" or 16 mm which is close-enough to 5/8" to not make a difference.
5/8 = 15.875mm so 0.125mm off
3.175% difference
not enough difference to cause a problem with the system
Remember that in the reverse-bleed method, you're allowing the natural tendency of physics to apply:
bubbles rise. With an open path via the pieces of the system, caliper, slave cyl, tubing, hose, and the MC, capable of passing fluid without impediment, and no leaks, I've found that for me, the reverse-bleed does the trick. You may have discovered that for your experience, and with the equipment you have and use, another method works well for you. Use what works best for you. If you encounter the "
*&%*$#@! system will-not bleed!" then try another method.
A word about the last statement, many people find that using either method discussed, a couple wraps of teflon tape on the bleeder valve threads reduces the introduction of air getting-past the cracked-open bleeder valve.
I open a bleeder valve no-more than one revolution from closed, that's all that you need. And be careful that when wrapping the teflon tape that you do not cover the bleeder hole in the unthreaded, tapered portion of the bleeder.
