Cleaning the Relief Valve
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Clean around the nipples on the valve so that no dirt can penetrate brake system from the pipes.
Place a rag under the valve to protect the painted surfaces against brake fluid spillage.