I’ve just completed a full front brake overhaul on my ’71 CB 500, aiming for peak performance and safety. The process included a meticulous cleaning and rebuild of the stock master cylinder, installation of new braided hoses and copper washers, and a thorough inspection of the caliper with pads that are practically brand new. I even bench bled the master cylinder before putting everything back together and, of course, used fresh brake fluid.
After what feels like an entire bottle of brake fluid bled through the system, I’m still not seeing any air bubbles whatsoever. The master cylinder is pushing fluid strongly, squirting a stream a good inch or two above the reservoir when open. I’ve adjusted the caliper multiple times and on dry land, things seem okay – I can lean the bike back and stop the front wheel easily with the lever.
However, my first test ride revealed a significant problem: weak and spongy front brakes. The lever simply doesn’t feel firm, and the braking power is far from confidence-inspiring. I’ve double-checked for leaks throughout the entire system, but everything appears dry.
Then, my dad stopped by with a vacuum Brake Bleeder. This tool has a pressure gauge, and that’s where things got interesting. When we pressurize the system with the vacuum bleeder, the gauge slowly loses pressure. Opening the bleeder screw with the vacuum bleeder attached immediately produces a stream of tiny air bubbles. This is confusing because with conventional bleeding methods, I get absolutely zero bubbles. We tried different fittings and hoses on the vacuum bleeder, but the bubbles persist from the bleeder screw, even for a while after tightening it. If we pressurize the vacuum bleeder and seal the hose, it holds vacuum perfectly.
Could this mean the bleeder screw itself is faulty and drawing in air, causing my spongy brake issue? And if so, does anyone know the correct size for a Honda bleeder screw? My Yamaha parts are too large, and local auto parts stores like Pep Boys haven’t been helpful.
I’m open to any thoughts or ideas at this point. Is a bad brake bleeder screw the likely cause, or could there be something else I’m missing in my brake bleeding process or system? Any advice on troubleshooting spongy brakes after a rebuild, especially related to the brake bleeder, would be greatly appreciated!
