radiator not holding pressure
 

I just purchased a 77 siverado with a 6.6 has 104,000 original miles. I have replaced the radiator,radiator cap, waterpump, both hoses, thermostat, and added a flex fan. My temp guage tends to go up to the red after i have first started it and then goes down to the middle of the guage. i have noticed that my radiator doesnt have any pressure in it even after running it for awhile. I havent ran a pressure test on it yet. Im not loosing any radiator fluid and there is no radiator fluid in my oil either. what could be the problem or what should I check next? thank you for any help in advance!

Question: Did you replace all of the componets because of the problems you have or did the problem persent itself after the parts were replaced?

the older chevy engines run what is called an "open system" after the thermostat opens there will NOT be any pressure...but a pressure check would rule out a blown head gasket.....but if you are not loosing coolant then i would say you are good there.

I noticed the waterpump was leaking when i first got it. Then after replacing that I noticed the temp guage running hot so I purchased a new thermostat (ac delco). Then noticed the radiator started leaking at a top seam so replaced that and new hoses and rad. cap. Then was still running a little hot so purchased a flex fan. The flex fan dropped the temp some. Im going to go purchase a pressure tester tomorrow to run a test to see where im at. Im new at working on these old trucks. Im used to workn on imports. The systems on imports have pressure all the time. I was wondering if all the old trucks like mine have the temp guage run all the way hot then when the thermostat opened up the temp. guage would go down to about half. I heard the 400 small blocks ran hot. Is there any way to drop the temp some more?

Does it have a fan shroud?

Yes it has a shroud.

So I was talking with a guy that ownes a radiator shop and he says on the old trucks he would drill a small hole through the edge of the thermostat since it was an open system and that would help with the cooling system problem im having. So i did and my temp. guage gets to half then drops down to bout a notch or two. where as it used to get to red line then the stat would open up, drop to half then climp a notch or two. So my problem seems to be fixed!

Well then your problem was air in the system. Those motors need to be idled for a while with the radiator cap open to circulate the air out. Also with the heater on. Sometimes even rocking the front end up and down will help push air out. Thats the only thing drilling those holes do, is allow fill up and push the air out. Glad you got her fixed!