The master cylinder banjo bolt is a special shouldered hollow bolt that makes a sealed connection between the upper brake hose and master cylinder outlet.
The 3/8-24 TPI UNF banjo bolt allows brake fluid to pass through the hollow bolt, transferring pressurized brake fluid from the master cylinder through the banjo fitting on the upper brake hose.
The banjo bolt passes through the banjo fitting and is sealed by copper washers on each side of the fitting. It is critical that the banjo bolt is in good condition to make a reliable seal at the junction.
A special copper washer is fitted to each side of the banjo fitting on the hose, and the banjo bolt is threaded into the master cylinder body.