How do you guys go about filling your rear master cylinder?

[OldGreyBeast]

Finally got my rear master cylinder rebuilt. The boot for the actuator rod did NOT want to print properly. That thing was a pain to draw and even more of a pain to get to print right... ANYWAY. Now that the MC is rebuilt and ready to go back on the bike, how do you guys go about filling it? There's not really room in there to take the lid off while it's installed, but I'd imagine if I fill it up off the bike it's going to drip everywhere... right?

What do you guys do?

[LAB3]

Prior to mounting it back on fill the reservoir with fluid. Cover the outlet hole with your finger and pump up some pressure until it squirts out around your finger, this "primes" the cylinder and takes one heck of a lot of pumping out of trying to bleed the system.

After you mount it up and start the bleeding process you will need to add fluid and like you said, there isn't much room. You could use a small funnel to get the fluid in there just make sure it's clean before you try.

[cajun31]

There may not seem to be enough room but that is how you do it. I would suggest that you spend a few dollars on the mity vac or something similar. Harbor Freight has options if you are near one. That way you can keep a vacuum on the bleed screw and you just keep the reservoir filled as you proceed. When you get nothing but fluid coming out then cap the reservoir off and bleed the brakes a few more times and then top it off with fluid and you will be good to go. Not having something like the mity vac makes bleeding the rear brakes a pain. It can be done, but after you have removed and replaced that reservoir cap a few times you will see that the vac system makes it way easier. Besides, you need to replace that fluid about every two years or so anyway. Good investment and makes bleeding the front way easier too.

[Larrym]

On my 79 Special? ( •_•)7

1. Remove RS sidecover.
2. Use small wrench to loosen each MC cover nuts one at a time.
3. Remove and store each nut in a tray/container. (Back to step 2 twice before moving on, right?)
4. Lift up and Slide out/away MC cap and steel plate/rubber seal.
5. Open fresh bottle of DOT3 Brake fluid.
6. Insert empty epoxy syringe and draw up 10 ml fluid.

Link for syringe(s) here-
https://www.amazon.com/Adhesive-Appl.../dp/B083YZ726C

7. Carefully add fluid until at proper fill line.
8. Empty any remaining DOT3 in the syringe back into the Brake fluid container.
9. CLOSE DOT3 container tightly with lid.
10. Throw the syringe in the garbage.
11. Reinstall by reversing steps 1-4.

If you don't follow step 10? Decide to Keep the syringe? (╯•﹏•╰)

Next time you try to use it you'll discover why some rubbers/plastics ain't recommended fOaR use wit' DOT3. ( ˇ÷ˇ　 )
(Spoiler- the rubber plunger will be totally stuck/immovable inside the darned thing.) (⊙_☉)

[Larrym]

Note- Did same Rear MC remove/rebuild replace on my special Oct 26, 2023. (◕‿◕)

IIRC, did the bench bleed of MC and then afterwards installed cap/gasket and nuts to prevent spillage during the next wrangling/wrestling it on the bike and into position. ( ⚆ _ ⚆ )

Also used an orange plastic "plug" (plus a little teflon tape...) in the MC outlet to "contain" that DOT3 during the install process.
d(⌒ー⌒)

Sorry fOaR using just words here. Didn't take photos to make a "Tech Tip" or anything like that. (˃̣̣̥⌓˂̣̣̥ )

[OldGreyBeast]

Thanks all! The syringe suggestion is perfect, I ordered those and will see how it goes My MC nuts are so rusted that they pulled the studs out (I actually thought it was long bolts that held the cap on). I'll have to see if I can get those apart or source some replacement studs. I can see how you could get the cap off with the MC in place if it has nuts instead of those bolts.

[cajun31]

Honestly, I have never had a problem just pouring fluid into the mc from the bottle. You have to go slow and be careful but I don't see the need for a syringe.