My company is actually a distributor for Festo and I too thought that push in fittings were not good under vacuum and have always wondered why some engine manufacturers supply push in fittings with their pumps for the suction side.

The other day I looked at the data sheet for push ins and found that they are suitable for vacuum down to -0.95 bar, absolute vacuum is -1.013 bar!

If all else fails read the instructions!

Malcolm