I had the same problem after replacing pads a few months ago and attempted to bleed the rear brake and ended up with mush (the front was perfect)
The issue was cause by the lack of height difference between the lever and caliper. I had a moment of insipration and hung the bike on a hook by the front wheel, repositioned the lever and bled it as per the Avid procedure – hey presto one rock solid brake that no longer went to mush when the bike was inverted.
Also – make sure you reset the pad spacing as per the Avid instructions (install pads, squeeze lever until they are a few mm apart and then insert the pad spacer between the pads to set the gap)
Like I said once I followed this procedure I ended up with perfect brakes. The draw back of Avid is that they require regularly maintenance to keep them tip top, something that I’m too lazy to do hence going for the new Shimanos. In fairness to the Avids though once they are setup properly they are very powerful.