Master Cyclinder Replacement Issue

[79XS11F]

I installed a NOS front master cylinder and am now ready to bleed the brakes. I did the them and removed all the air that I could find in the system. No air coming up in the MC and no air coming out of the bleed nipples. Once I had removed what I think is all the air the old fashion way I power bled the system with a Vacuum bleeding pump.The brake lever however is not developing any back pressure. I am hoping that the seals in that MC need time to become soft and seat properly now that they are in contact with brake fluid. Manual says that if bleeding is difficult that one should wait a few hours for system to stabilize. I waited over night and noticed that the fluid level in the MC was a lot lower then when I went to bed. There is no leaks in the system that I can detect and no brake fluid in the caliper piston boots. Being that this MC was NOS how long do you think I should wait before switching it out for the working MC that came off the bike?
Rob

[John]

Bleeding the front brakes on these bikes can be a very frustrating undertaking. Just last week I swapped out my front calipers, and I couldn't get a good 'pedal'. I ran about two reserviors worth of fluid through each caliper, so I felt good about each side getting a good amount of new fluid to it. When I tested them, nothing... the calipers were still compressed in their bores. There was still some air some where. I have a tool, a 'Phoenix Injection System', that is designed to pressure bleed the system backwards, from the caliper to the M/C. After forcing about two ounces through each caliper, I still had nothing. My solution was to remove the M/C from the handlebars and hold it so that it was rotated 90 degrees from it's normal position. I had noticed that with my particular setup, that the banjo bolt that attaches the hose to the M/C was actually the highest point in the system, and therefore if there was any trapped air, it would be there. I had Wildkat hold the M/C for me, and I injected a few more ounces up through the lines, and apparently I was finally able to chase the air bubble out of the system, as I was now able to get a good, firm squeeze on the lever, and the calipers started expanding.
If you don't have an injection tool, then one way to chase the air up to the M/C is to remove the caliper from it's mount, and using a small C clamp, you can compress the piston back into the bore, and force the fluid up to the M/C that way. That, combined with turning the M/C 90 degrees, should chase any air from the system. In the next few days I plan on swapping out the rear caliper, so we'll see what trick is neded to bleed the system.

[79XS11F]

Thanks for responding John. With my standard setup the MC reservoir is in deed the highest point. I again tried to get this MC working but NO LUCK. I got fed up and switched it out for the one that I rebuilt last spring and had the brakes bleed and working in 15 minutes. I guess I will have to take this NOS MC apart and check out the insides Perhaps the guts just need to be soaked in brake fluid. I am guessing that that may be the issue because i know that when I did my rebuild the instructions said to soak all the inner seals in brake fluid for at least 15 minutes before using. Perhaps in the pre-assembled form only the leading edges are being fully exposed to the fluid. I will do that when I start work on my hop up bike.
Rob

[beegshot]

Depending on how your MC is...

oriented on the handlebar (really depends on the shape), you can achieve a good result with a vacuum bleeder if you turn the bars to the left (effectively elevating the MC and raising the pressure bleed off orifice in the bottom of the MC above the banjo fitting). But, if your bars are turned down alot, you'll have to remove the MC as stated above.

My $0.02.

Robert