Your grandpa is suffering from recurrent pleural effusion
for which ther are many cause and i am sure doctors must have told you the cause in his case. A pleurodesis
has been done in his case to obliterate the pleural cavity
so that fluid does nost accumulate. If no more fluid is coming in the bag it is quite possible that the procedure has been successful and no more fluid is forming and what ever fluid was in the tube has solidified because it is very high in protein content. But there are also chances that the tube is blocked or it is not in the pleural cavity. Does the fluid column in the tube move when he breathes? You need to go back to your doctors and they will do the needful.