1978 Other Chevrolet Models

You probably didn't bleed the system correctly. Have someone pump the master cyl then break the front brake line loose til you see the fluid come out then close it and have the person release the pedal. Then do the rear brake line the same way. Once there is fluid coming out both go bleed the brakes the rear first- right rear then left, right front then left. Do not let the master cylinder run dry. If this doesn't cure the problem, take it to a pro or rent a brake bleeder from a rent it shop if there is one around and use that.