Having some issues with the 4L80E in my '98 K2500. The truck has 265k miles on it and I'm not sure if it's the original trans but I'm sure it's got a lot of miles on it because just after I bought the truck I had to replace the pump pressure spool valve (used the Sonnax kit) due to high line pressures in reverse. If that was worn out I'm sure the trans has a ton of miles on it, but that was 3 years and a couple thousand miles ago and the truck has been pretty reliable. Here's what it does and what I think is going on. The truck has an intermittent transmission leak. Sometimes it will go weeks without leaving a puddle or needing fluid, and then sometimes it will leave a puddle while sitting or leave a big smear under the p/s frame rail after a drive. I initially thought it was the cooler fittings in the side of the trans leaking from where I had to pull it for the boost valve replacement, but now I'm not sure because of the other thing it does. If you drive the truck regularly, like at least every few days, the trans shifts fine. If you let it sit for more than a week or so the trans will not go into gear at first when you start it up. Give it 5 or 10 seconds in drive and afterwards it will shift fine all day and will be fine the next day as well until you let it sit for more than a week. Seems to me like the converter is draining back into the pan, or possibly my external leak is related and the converter is leaking fluid externally somewhere. Anyone know where the check valve would be to stop converter drainback? I'm thinking that either my converter itself might be leaking, or that I do in fact have a cooler fitting that's leaking and it's letting fluid out of the converter and onto the ground? I don't like having the converter starting empty so often, not as concerned about the mess or keeping an eye on the trans level. Maybe someone else has had an issue with converter drainback? I will also note that the pump in this trans showed some definite wear, although it continues to make spec pressure. I suppose my drainback could be through the pump, and the leak completely unrelated?