turbo2

My NV600 transmission rear seal was leaking into the T- Case. 250,000 miles. When I checked my fluid levels, I discovered I was losing tranny fluid and gaining T- case fluid. Lots of fluid! I had overfilled my tranny as many do on this forum. I dropped the T-case and discovered the old rear seal on the transmission had gotten brittle and hard and was letting fluid out of the tranny. I put a new CRW 19211 seal in it. While I was at it I thought why not put some new seals in my T-case too. So, when I took them out they were still soft and pliable and in pretty good shape. I did replace the single lip original national seal with a NAPA 19255 double lip seal, which looks like it will help in the future if the rear tranny seal fails again. This double lip seal is the input shaft seal up front on the T-case. I should add that my tranny and my T-case have rtv silicone sealing then together. Thats probably not how it came from the factory, but I put it back together with rtv because it seems to be a good idea to keep dirt and water out of there and now that I have the double lip seal on the input shaft, the tranny fluid will have to get past the first seal on the tranny and then past the second seal on the T-case. So, my question is this, does anybody know why the rear seal on the tranny gets hard and brittle? Is it the manual tranny fluid? Or? And should we really be overfilling these NV5600 trannys? Over filling these trannys might be a really bad idea if it causes seal failure, but maybe it doesn"t. Any ideas from any of you mechanically gifted gurus?

KATOOM

Yes, heat is the common cause for seals to get brittle but I guess that doesn't mean one cant just be from a batch of bad seals or was installed wrong on the assembly line. Did you buy your truck new? Did you ever change the fluid and if so what did you replace it with? Do you tow and if so how often and how heavy?

turbo2

I bought the truck used at 170,000 miles. It had original fluid, so I changed to amsoil and overfilled. Now I have added a little penzoil sycromesh to top it to the plug level. I ahve only towed a few times. I am stating to think the plug level is a safe level to fill to. If a rear tranny seal fails and I have over filled then I will fill my T-case to the top with fluid, which can not be a good thing and might case T-case failure. IF it leaks out the front then I soil up the clutch which leads to clutch failure. I do not think I am giong to overfill from now on, and see how that goes.

turbo2

I got to thinking, always a dangerous activity, the rear tranny seal and the front T-case seal serve side by side in the same enviorinment, same temps, same job, same everything except, different fluid. In my truck they are within 2 or 3 inches of each other. Both get fluid thrown up on them and unless you overfill the tranny, neither has to hold back a lot of fluid pressure. So, why is the T-case seal soft and pliable and the tranny seal all hard and brittle? Is the fluid that different? I have a temp gauge on my tranny so I know it has never been above 185 degrees. Any ideas out there?

KATOOM

I dont think that over filling would do any such harm. The extra quart isn't going to place any excessive pressure on the tranny if the vent is clear. Its just a way to get more lube capacity. If I was you, given you have no previous history on the truck, I'd just replace the seal and chalk it up as a fluke. Some have T-case front seals go bad and some dont. Some have output shaft seals go bad often and some dont. If it bothers you that much then start calling some reputable tranny shops like quad4x4.com or allstategear.com or, I think there's another good one too but cant think of it.