If you hold in the button, the depth finder mode takes effect, which is actually a motionless all-metals mode that can be used to find your object even if you don't know anything else about how to set the detector up.
Turn your detector's DISC knob all the way down. IIRC, if you have huge amounts of iron in your soil, use GEB Max, but normally just use GEB Disc. Put everything on preset (the arrow that has a P in it) except the DISC knob, which as I said, turn it down counterclockwise, so the pointer is at or below "nails" and well below "tabs". I think you'll want the DISC knob down around 2 or 3. However, a large iron pipe should still show up even when DISC is turned up to medium. Especially if the pipe is rusted. Like the other poster said, you HAVE to keep the coil moving... make wide sweeps, back and forth. UNLESS you hold in the button to get that non-disc / depth finder mode. That works even when not moving.