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I have a 98 Astro van with the 4L60e transmission.  It failed and I had my local mechanic rebuild the tranny about 4 months ago.  Then about a month ago it started slipping and the fluid was low.  Added fluid and then I had a leak at the front of the tranny.  Took it back to mechanic.  They pulled it and replaced the front seal.  The seal looked OK but seemed like the location of the leak.
It drove fine for about 1 week, then started leaking again just like before.  If the seal failed again is there something that is making it fail?  
Thanks for any help!  

Repeated front seal failure could mean that the pump bushing was not replaced during the rebuild but that is not likely.The pump can also leak through the pump o-ring around the outside of the pump.Also there are little o-rings on each pump bolt that have to be replaced.Another possibility is that the vent is clogged and there is pressure building up inside the trans.Do you have the kind of dipstick that locks on? If you do try pulling the dip stick out of the tube an inch and leave it that way.If the front seal stops leaking then you have pressure building up in the trans.Also check the flywheel for cracks .They usually crack near the center where it bolts to the crankshaft.  

Insufficient fluid drain-back from the pump can also cause the front seal to leak. If any upgrades were made to increase pump pressure the drain passage should be drilled out to 1/4 inch.  

440 is pretty right...  99% of the time if the front seal is leaking the bushing is bad.  may not look it,  but it will be lose and walking out and closeing off the drainback hole.   or so worn the drainback hole can't do its job.  
repeated bushing failure is usualy a bad converter (hubs not true)  or a damaged flexplate/crank pilot.