Okay - so I started tearing into the rear brakes to see what the problem is on my new truck. It is a 1984 c20 with a diesel motor. The RPO sheet is missing but the door tag says it has a GVWR of 6400lbs. It has a 14B SF and 11-5/32" by 2-3/4" brakes. When I went to the store they listed JD6 or JD7 as the brake options, since I've got the smaller brakes I went with wheel cylinders for the JD6 - but it doesn't fit. The wheel cylinder I took off looks more like the 13" brake version (JD7?) Anyway, is it possible to figure out the RPO codes for the truck by the VIN? And where can I get the little bolts that hold the wheel cylinder to the backing plate - these are trashed?