Motorcycle Repair: engine just wont start, suzuki gs 750, leakdown test

Question
What is a leakdown test and how do you do it?
Followup To
Question -
I've got a 1979 suzuki GS 750.  Did a compression test on it, and found the rings needed to be replaced.  I did that, assembled everything back the way it was before, got ready to start it, and it just kept turning.  Did another compression test after, and the cylinders averaged between 30 and 60.  Lower than the 1st test.  I just can't figure out what went wrong.  It has spark and fuel, but I can't figure out why it has no compression.  Any help would be great. Thanks.
Answer -
Hi Matt,

The cylinder walls and/or valves & valve seats are worn.

Perform a leak-down test to help determine the source of the leak.

Measure pistons, cylinders, and other parts to know if they are worn out of service limits.

Respectfully,
Mark Shively

Answer
Matt,

Sorry for the delay. I've tried replying several times over the past two days. AllExpert's servers must be down. I have 38 questions unanswered that I didn't know about!

Anyways, send me your email address as requested in my instructions. I return tech files by email. I'll send you the scoop on leak-down, and more.

Respectfully,
Mark Shively