I had the same problem on my 931A2. Changed all flexible fuel lines, tightened & sealed all fittings, replaced the O-ring on the fuel selector valve, but it still lost prime. Replaced the priming pump on the engine, checked both fuel filters (had just changed them several months before & driven the truck afterwards with no problems), still had same problem.
Then the driver's side fuel return line started leaking fuel at the elbow going into the tank...I understand that 923s have only one tank, but the pieces/parts are similar. I removed the fuel return line and found that it had been overtightened & cracked along the flare on the tubing. I cut off about 1/2" of the copper return line end and reflared it. I don't remember what prompted me to remove the elbow fitting going into the tank, but I'm glad that I did. I expected to find a solid tube return line from the elbow down into the tank, but it was a flexible fuel line! Further, this fuel line was obviously not rated for fuel immersion as it was swollen to the point that little to no fuel was getting through it AND the exterior was crumbling away, dropping chunks of rubber into the bottom of my fuel tank. The flexible fuel line was crimped to the bottom of the elbow, so it was intentionally used there.
My truck was a 2005 RRAD rebuild. I replaced the elbows with new ones that had a new SOLID tube brazed to the bottom of the elbow, 16-inches total overall length. The TM -34 gives a part # and NSN for the return lines, and specs it as a solid tube, NOT a flex line. I searched the interwebs for the NSN and found some 'nominal' specs for overall length, nowhere could I find any info on using a flexible fuel line.
I checked my primary fuel filter, and sure enough found some black rubber chunks in the canister housing. I cleaned these out, and flushed the fuel line with compressed air to blow out any chunks inside. A few came out, so I improvised a fish tank net with extended handle to reach down into the fuel tank to retrieve the larger rubber chunks from the disintegrated hose.
PROBLEMS SOLVED. I know that the return line shouldn't affect the supply line prime, but my loss of prime problems disappeared after this. Now it fires right up & holds its prime no matter how long it sits.
