We present high resolution observations of SiO masers in the late-type star R Cas as part of a multi-line study of SiO circumstellar masers in oxygen-rich AGB stars. We used the NRAO Very Long Baseline Array (VLBA) to map the 43 GHz maser emission of 28SiO. We study the relative spatial distribution of the different maser lines and discuss the relevance of line overlaps in the SiO pumping theory.