Brake help needed, 68 subject c10
 

My father-in-law and I just put a new master cylinder/booster combo on the ol truck (Summit Racing part # tff-2128nb ). Got the Mc bled just fine but cannot get a firm pedal. Plugged ports in Mc and pedal is very stiff but once we hook up brake lines pedal is very soft. Even softer when engine is running. Have drums all around and new wheel cylinders on rear since we weren't getting as much fluid to the rear. There is fluid to the rear just not as much as the fronts get. Is it my brake lines running to the rear ? Any help is appreciated. Ty in advance.