QuestionI have a '78 CB400A Hondamatic and I recently found an OE style round master cylinder for it. When I received it the piston was froze. I was able to finally unstick the old piston and remove it from the cylinder. The bore looks good as best I can tell and I bought a new kit to put in it. I put it back in the way the old one came out and I also followed the image on this website. http://www.hondapartshouse.com/#/Honda/CB400A_A_%2878%29_MOTORCYCLE%2c_JPN%2c_VI It shows how the kit should go. The first time I put the kit in I put the cup in the direction it shows and the open plastic piece against it. That didn't work so I looked at the old piston and sure enough the open plastic piece is on the piston itself. So I took the kit out and put the open piece on the piston and put the master cylinder back together. I should note that the first time I tried bleeding it with the open piece not on the master cylinder fluid came out the other side near the handle. However now it is not leaking around the handle and I've pumped for about an hour tonight. With my finger over the banjo bolt hole I can feel air push against my finger (not much) yet I feel a lot of suction pulling my finger when the handle is released. I read a tip where I tie the brake handle down overnight. I'm doing that tonight so that air bleeds by itself; However I would also like to know if judging by the picture if you think I installed the master cylinder kit correctly and if you believe that is the proper direction for the cup? Please advise on any ideas as to why the master cylinder will not bleed directly at the end where the brake hose goes. Thanks in advance!
AnswerJames, The piston cups all have to "scoop" the fluid forward towards the outlet when the lever pushes the piston forward.
If the bore has any scratches, etched areas where corrosion ate into the metal, then it won't hold/build pressure properly.
Both of the holes in the floor of the master cylinder must be clean and clear. The smallest one forward is the bleed port, which releases the pressure when the lever is released and the piston retracts due to the spring pressure.
When you are trying to bleed the brake cylinder DO NOT pull the lever all the way to the handlebar grip. You need to put your finger or a 1/4" spacer between the two, because the lever will over-travel the piston past the fluid ports, which does nothing to help the bleeding process.
Tying down the lever may work, again if not pulled all the way to the bars. If fluid is leaking past the piston, spacer/ snap ring and dust boot, then there is damage to the cylinder bore and you are wasting your time trying to bleed it.
When bleeding the master cylinder, always keep the fluid above the bottom of the floor of the reservoir, so it doesn't start sucking air back into the system again.
Bill Silver
