I stopped worrying about getting a perfect seal a long time ago. Between milking the tube and kegeling and wrapping heat pads, there is a lot of tube movement and that can cause some leakage. Also, as it turns out, it is better to pump with a varying pressure anyways. Holding a constant pressure just leads to blood stagnation and that leads to discoloration. As long as you don’t have major leaks, I wouldn’t worry about it and major leaks are usually found due to a leaking connector. I wrap the threads of the connector that screws into the cylinder with Teflon tape and that seals very well.
As far as lube goes, I find it really takes just a little bit to make a good seal at the base. I find the lube to be more important to be applied all over your cock so it doesn’t get hung up inside the tube. I always thin the lube after I apply it with some warm water to keep it really slick before entering the tube. I have never used astroglide so I am not sure if it is as thick as the KY equivalent lube I use from Target of Wal Mart. The Target or Wal Mart brands go for less than $2 a tube, so you can save a little money there. :)
I have never had a burping sound, but that sounds like a leak between the cylinder and your skin. Are you applying the lube all over your cock? That will really help with a 360 degree seal at the base as your expand.