That is odd!
Unless the reservoir was overfilled to start with, my only guess is that when the cap was removed, it jarred loose whatever was jamming the master cylinder piston. What happened when you put the lid back on?
2-04 R1, 81 CSR1000, 81 LTD1000, 2-83 GPz1100, 3-79CBX, 81 CBX, 3-XS650, 84 Venture, +parts
Quote "speed costs money...how fast do you want to go?" (Which Z movie?)
Universal formula for how many motorcycles one should own = n + 1, where n is how many motorcycles you own right now....
