We look for repeating earthquakes at Shishaldin volcano (Alaska) by a multi-stage clustering of the waveforms of a large catalog of Long-Period (LP) earthquakes (about 330,000 events) occurring between October 2003 and July 2004, that includes a minor eruption. We find about 5,550 LP earthquakes with at least on repeater in the catalog; they mostly occur in the interval December 2003 - March 2004. Repeaters are composed of two ∼5-s wave packets. The first packet is a direct source effect and its polarization dip angle points towards the nucleation depth of the pressurization phenomenon triggering the conduit resonance. The second packet is in the coda of the first one and is dominated by scattered waves. Repeaters are LP events with common pressurization position and triggering mechanism. The nucleation depths of the overall LP seismicity and of the repeaters (inferred by the polarization dip angle) show a sharp increase in January 2004 that temporary matches the onset of the eruptive crisis, followed by a slow nucleation depth increase in January-May 2004, that we interpret as a fast pressure drop followed by a slow pressure build up in the feeding system. During the latter stage, coda wave interferometry applied to pairs of repeaters indicates a medium velocity increase. By simple macroscopic modelling of the feeding system as a homogeneous porous medium and a velocity increase as due to squeezing of fluid-filled cracks, we estimate a pressure increase of 40 MPa in Dec 2003 - Apr 2004.