Transfer case leaking 85 K20

[silverchev08@gmail.com]

So my transfer case leaks a little bit but I can't figure out from where. Do you think it's better just to replace it or take it somewhere to have it rebuilt ? What's the fix?

[Irish_Alley]

clean it up with brake cleaner or something then try to find the leak. might be a seal or might be a crack

[hatzie]

There's not much fluid in a transfer case.  Around 2qts max. 

If you run it low on fluid for very long you'll get to replace it.

Clean it off, fill it up, and see if you can find the leak without any additional work.  There are UV tracing dyes that you can add to the oil.  UV dye makes the leaks show up under a black light like a neon sign.
Make sure you use the right juice in it.
NP203 used SAE30 Non-Detergent
NP205 used gear oil or Dexron III ATF depending on the year.  Check the owner manual.  I used to use 75W90 GL5 Synthetic in my 78 K20.
The NP208 & NP241 used Dexron III ATF. 

DO NOT USE DEXRON VI in any of these transfer cases.  GM Tech Service Bulletin 04-07-30-037E says to use GM 88861800 or AC Delco 10-4033 in the transfer cases and transmissions that originally used Dexron III.  It's $6.99/qt at Summit right now.  The Valvoline and Mobil Dexron/Mercon "compatible" fluids were around $15/qt the last time I checked.

[JohnnyPopper]

I thought they stopped making the NP203 in '79?

My '78 says to use 10-30 multi grade, nothing about non-detergent.

Is that why it leaks Hatzie? 

[hatzie]

I thought they stopped making the NP203 in '79?

My '78 says to use 10-30 multi grade, nothing about non-detergent.

Is that why it leaks Hatzie? 

If the plugs are tight, seals are good, and the case is in one piece it shouldn't leak no matter what oil you put in it.  The chain and bearings and diff might not like it if you use a heavy or super light oil but it won't leak. 
IIRC My 76 K20 full time 4WD called for 30W non-detergent in the transfer case.  Not sure why they speced non detergent.  Probably had something to do with the differential between the front and rear outputs but who knows.
If your owners manual doesn't say non-detergent and says to use 10W30 then use it...  10W30 will essentially be 30W at operating temp and lighter when you first startup when it's colder out. I wouldn't put heavier 10W40 or 15W40 or lighter 5W20 etc in it but that's about all I'd worry about.
There are tales of Synthetics causing leaks in older machinery but IMHO that's just bad seals and the like rearing their ugly heads when actual maintenance is done.