GMC Repair: no heat, rad cap, truk

Question
1989 gmc 1\2 ton 5.7l truck .i have heat to core but not in core our out pipe back to radiator .i can blow from radiatorall the way back through system w\ my mouthand out through rad. cap.  how can i make my core get hot fluid through it?do these cores block and if so why can i blow backwards through system/?

Answer
hello brian
try filing rad to top . leave the rad cap off then start the truk up let idile on till the thermostiac opens then race the eng. till pulls the fluid down in the block then shut off fill keep repeating it and till u get heat this is called burking the system try it it should work pete