We are talking clutch here, not brakes.

I replaced the "cup" seal in my slave a couple of months ago. When I removed it, there really wasn't any visible sign it was leaking. There wasn't a sign brake fluid was leaking anywhere really.

But now I notice it is starting to lose proper lever feel again--it will want bleeding again soon.

Do I just replace stuff until it stops or is there something a real mechanic would do to figure it out properly? (If the former, what should be replaced next?)

BTW, this wouldn't have anything to do with me using DOT3 fluid, would it? Doesn't DOT4 suck up water?

Thank you.
sean

dot 4 is the recommended fluid,by using 3 that may be part of the problem

i'd flush it and replace to start

on mine i seem to have to add a small amount of fluid about once a month,until it gets worse i'll leave well enough alone

valvoline and castrol both makes a syntheteic brake fluid that is compatible with dot 3 and 4 at the same time,been using for years w/o a problem

Dot3 and dot4 are virtuslly the same. The main difference being boiling point. That's why synthetic brake fluids are designated 3/4 compatible. Don't want to talk about dot 5, whole different animal.
Since the clutch doesn't see the extreme heat brake systems do, dot 3 will work just fine. If you look at the specs, the boiling point for dot4 is only marginally higher than dot3. In my humble opinion, anyone getting their bike brakes hot enough to boil dot3 under most driving conditions, needs to park it and stay off the road. Honda went to dot4 for their brake systems. It wouldn't make much sense to have three reservoirs using different fluids.
Are you loosing fluid? I know you don't see any leaks, but is the reservoir level dropping? If it is, it's going somewhere. Find the leak and fix it.
If the level is constant, the next step would be a master cylinder overhaul, if you feel you did the slave cylinder correctly.
Also check the brass "barrel" at the clutch lever. Most people don't know it's there. It's probably worn out. Strange thing is, if you lube it yearly, it lasts forever. Maintenance is a wonderful thing.

just a passing thought,maybe you have a crush washer in the system losing pressure somewheres

i have soemwhat of the same problem as mentioned before,livable but aggravating

Silly me. I was thinking that if I had a leak, fluid would be going somewhere.
(And it may, but where I can't see it.) But it was TOTALLY off my radar
that a leak could merely be letting air INTO the system and of course THAT,
air in the system, is my real problem (unless fluid gets too low, of course).
Stuff like this is why nobody pays me to work on their bike. :)
This is a lot slower occurring than before the cup replacement, so I'm
not sure if my level is dropping. I'll bleed and check and ponder.
Thanks!
sean

If the drain hole on the rear cover is plugged tight then it is possible for brake fluid to drain into the engine and you'll see no leak on the outside. Make sure that when you bleed the system that you bleed at the banjo bolt at the master cylinder. Also, if the lever bushing is worn out badly it may be possible that you cannot bleed properly. The lever bushing is easy and cheap to replace.

My fluid level is dropping, but I see no leakage at the master or
slave (externally). Is there no science to this? If I were a mechanic
charging somebody $60+ an hour, would I simply tell them I need
to rebuild/replace their master & slave or could I actually pinpoint the
problem somehow? This makes me feel as though my long motorcycling
life has been for naught. :) What have I learned? Apparently not enough
to fix my Aspencade's brakes.

Beth, my clutch has a slow leak also and, as the leak is not at the clutch lever/master cylinder, it has to be at the slave cylinder bleeding internaly into the engine as Vic stated. It's a very slow leak so I've not gotten around to troubleshooting it yet, but I'd say that's where it is..

One of the main functions of brake fluid is to absorb moisture. That is part of preventing rust in the system. You should replace brake fluid once a year under normal conditions.

I would think what a (reputable) mechanic would do is first flush and fill the system with manufacturer recommended fluid, bleeding the system at the same time. Charge the customer for time and material, and send them on their way with instructions, ‘keep an eye on the level and performance and if the level goes down or performance diminishes, bring it back and we will take another look at it.’ This gives the mechanic a starting point as they should not trust what the owner (or their friends) have done. If the bike comes back with the same problem, then I would guess he would look for leaks. Not finding any external leakage, take the slave off the back of the bike, leaving it attached to the system, and test for leakage. I suspect the leak to be found there.
The recommendation to the owner may then be a rebuild of the master and the slave, eliminating any chance of future failures. Of course, the owner would have the option of doing both master and slave or just the slave.
In any case, that is what I suspect the mechanic would do.

Flush and bleed the system - 1.0 hour labor plus parts (since the owner may have already done this step, it may be wise to change washers at each banjo bolt.)

Service (rebuild) slave, bleed system – 1.0 to 1.5 hours labor plus parts (HONDA parts please)

Service (rebuild) master and slave, bleed system – 1.5 to 2.0 hours labor plus parts (again HONDA parts)

Just my $.02