Noob with a 12v 5 speed 3 gear grind
 

I just bought a 1998 club cab 4x4 3500 12v 5 speed with 242k. the NV4500 will grind when shifted into 3rd quickly, but only after the trans is warm, does not do it cold, and if you double clutch it it has no grind. Additionally it is very hard to get into reverse,the truck has a south bend dual disc clutch. I have noticed that with no foot on the brake, when you put itinto any gear it will want to creep a bit, like the clutch is grabbing slightlly. I'm thinking its a master or slave cyl issue, but wanted some other opinions.

nv4500 are notorious for sync issues and 5th gear nut backing out and cause it to stick in 5th, if its doing it only when you try to speed shift it then it sounds like your clutch isnt in its prime, and if its creeping with the clutch in, then yes, your clutch has seen its betteer days, but if it happens to be the syncs dont keep slamming 3rd, it will be completly gone soon, dont sound like master or line related to me

5th gear nut is fixed, and if the clutch was worn out it would slip when under a load, I just towed a 17k load in the mountains, and no slip, it's not a worn clutch.

Wondering on the creeping in gear, mine s
Does it too. Single Disk but it Never grinds!

In full disclosure I have never driven a nv4500. However, every manual vehicle I have driven has shifted into reverse easier if you shift from second to reverse.
In other words.
come to a stop
clutch in
shift into second then into reverse
This will usually cure all reverse grinds/ clunks etc?

I had a 1987 Supra in which I put a Centerforce Stage 2 clutch. The Centerforce loved double clutching. Have you asked Southbend if that is normal?

Unsure of creep
DD

I would check throw on the slave cylinder, and if thats good, change fluid in trans and look for metallic flakes , and what color they are,.... brass will look yellowish golden, steel will look silver, aluminum will look bright and shiny , in fluid of course ...... change out with a high grade synthetic gear such as RedLine or Royal Purple ...

I agree with changing the fluid. If there was a master or slave issue, it would affect all gear changes.

The majority of the problem is operator error. These transmissions were not meant to have their gears slammed. Shift like you want it lo last and not like you are trying to break it, and it will work perfectly. Not trying to sound like an @ss, but that's the simple truth of it.