Where could these 'beads' blocking expansion valve be coming from & how do I get it fixed?

Mechanic found 'hard little beads' in air conditioning lines which keep blocking the expansion valve. The lines were cleaned along with the expansion valve (up to where crossed to back of vehicle for the rear air). It blew cold briefly & then hot again. Expansion valve & codenser (which was bad) were replaced before these 'beads' were found in lines. Cost a small fortune already & still no cold a.c.! Garage said didn't know what else to do (this was the second garage to look at it). Thanks!