IF your chassis is a 2005 model year P32 series Workhorse chassis you will have the J72 version of the troublesome AAPB. It is a spring-applied, hydraulically released brake mounted on the DS just behind the tranny. Your best source of parts and info is at:
www.urvp.com The J72 is an improved version of the J71 mentioned above , and the parts are similar but not necessarily the same.