I wouldn't count on pressure pushing the seal off. Generally when a seal migrates, it's improper clamping.
When you do a compression test, remember that the compression value you get isn't particularly important, and doesn't tell you much...but what you hear while you're doing the test tells you a lot. The compression test does record a value, but it goes up and down with the cylinder temperature, test set, person performing the test, way in which the test is performed, etc. What the compression test doesn't record is the sound you hear of air escaping the induction (intake valve leakage), through the oil cap (crankcase/ring leak), or through the exhaust (exhaust valve leak).
Make sure you have a clean surface underneath the pushrod seal. Some like to lubricate the seal and the pushrod surface with oil, or other lubricant. In this case, the seal may migrate anyway, and excess pressure to prevent it can damage the pushrod tube itself. Keep it dry under the seal, use moderate clamping pressure, and where more than one clamp is used, have the screwpoints on opposite sides of the tube.
It's not uncommon to find the pushrod housing out of round due to previous overclamping. If this is the case, you'll never get a good seal except by accident, and you can expect the seal to migrate. Check the housing (pushrod tube) thoroughly before reinstalling.