Dog, welcome to the forum You say you have air bubbles in the pump, is there a chance that the pipe itself was breached allowing the concrete in, in the first place? If so, have care removing the obstruction as you might cause a fatal leak in the pipe (using power equipment or excessive force is also not a good idea as the pvc pipe will probably give before the concrete). If the crete was just a glob that ended up in the pipe a good poke should have freed it, I fear the pipe was compromised before the crete was poured/ shot (pvc & crete don't bond very well).
I would suggest keep doing what you have been (I know, pain in the @$%& and all those trips to the dive shop to refill). If you want to get a hammer and 1/2" cold chisel involved, have someone topside hold you down with your wallbrush, but be careful about the pipe. Once you get it down to ~ 1/4" mix up some 2 part epoxy (any plumbing store will have some) and generously coat the entire area, making sure that you have a good layer over the remaining concrete and adjacent pipe. Wish I could offer a better solution, but I believe this will take care of the problem. GOOD LUCK! - Waste