Where the flexible brake hose meets the caliper on many vehicles, you will find an odd hollow bolt called a banjo bolt. It is a clever fitting, and getting it sealed correctly is critical to safe brakes. Here is what it does and how it seals.
What a banjo bolt is
A banjo bolt is a hollow bolt that passes through a round hose end fitting (the fitting looks like a banjo, hence the name). Brake fluid flows into the hollow bolt through cross-drilled holes and on into the caliper - it lets the high-pressure brake hose connect to the caliper at a compact angle. It is common on the caliper end of the flexible hose.
How it seals: crush washers
The banjo fitting seals with two soft metal crush washers - one on each side of the banjo eye. When you torque the bolt, the washers deform slightly to seal the high-pressure fluid. These washers are single-use: you must fit NEW crush washers every time the banjo bolt is disturbed, or it will weep - a classic cause of a slow leak; see brake hose leaks.
Common mistakes
The big errors are reusing old crush washers, forgetting a washer, over- or under-torquing the bolt, and pinching the hose so it kinks. Any of these leads to a leak or a restricted hose. Torque to the manufacturer spec, and make sure the hose is not twisted when tightened.
Why it matters
A banjo bolt is a pressure joint - a leak here means losing brake fluid and braking; see brake fluid leak signs. Always use fresh crush washers, torque correctly, and bleed after. When servicing hoses or calipers, refresh worn pads and hardware at the same time.