The brake system on most vehicles consists of a master cylinder that forces hydraulic fluid through brake lines and into brake calipers or wheel cylinders. The brake lines are connected by a metal union. One side of the union has a threaded male piece and the other side is a threaded female piece. The two parts come together to form a seal. Over time, the seal can become weakened and a leak from the union may develop. Fortunately, you do not need to replace the entire brake line because it can be easily repaired.
Loosen the brake line union, using one line wrench to hold one side of the union and another line wrench to loosen the union. Pull the brake lines apart from each other.
Clean the onion threads thoroughly with a clean shop cloth. Place a thin coat of hydraulic line sealer on the threads of the male union fitting.
Reconnect the unions and tighten them together using two line wrenches---one to tighten and the other to hold the union steady.
Do not use Teflon thread tape to attempt to seal the lines, because the tape can come apart and cause damage to the brake system.