Feels like a fan clutch to me. Had the same problem with my Stroker....new clutch was defective....put another clutch in.....works perfect.

My 76 has done the same thing in the past.  It's either the fluid heating up and expanding or the water pump collapsing the lower hose, reducing overall system volume.

The coolant reservoir level will only drop as the engine cools off and a vacuum is created in the coolant system, pulling coolant through the tube attached to the radiator neck.  The radiator cap needs to be one designed to work with the reservoir, not an old 1950's cap.  And it needs to be working correctly.

If the radiator is full now you know there are no air pockets in it.  Put the cap on and leave it.  Make sure that the hose from the radiator fill neck to the reservoir is tightly sealed at both ends and has no pinholes, otherwise the system will suck in air instead of coolant from the reservoir as it cools down.

Jeremy. Is that the stock fan on the engine. Can you get a better picture of the fan. Something looks a little " Off "

If there is air trapped somewhere the best way I have found to purge it is start the engine cold and jack up the front end of the car with the rad cap off, make sure your interior heater valve is wide open. Usually burbs just after the T stat opens.

Edited October 29, 20177 yr by grannyknot

Does the engine run normal when driving (moving) down the highway? 

Thanks all for the help and suggestions.

It's the stock fan. I cleaned/painted it bright white a while back. I'm unable to drive it on the road right now because it's not registered or insured yet. I'll get it out if I have to, but I'd really like to avoid it if possible. I agree, though. I'd love to know how it acts while driving instead of just idling in my backyard. The system did pull coolant back out of the reservoir bottle after it cooled down yesterday. The coolant was an inch or more over the MAX line when I shut it off.

I should have raised the nose end up to purge out any air. I'll try that today. If it still gets too hot, I'll order another fan clutch. I remember buying the cheapest one ebay had a few years ago. Rock Auto has an AISIN unit for $36.

Just saying.....if it runs cool when moving, but overheats when at stop lights or just idling, it indicates that your air flow is adequate will moving, but not while idling (fan clutch). If it overheats while moving, you’ll need to look further.

On 10/29/2017 at 5:26 AM, jeremy93ls said:

Thanks all for the help and suggestions.

[Snip] It's the stock fan. I cleaned/painted it bright white a while back.[/Snip]

Ah...it was the bright white paint that was throwing me off. Didn't look like a stock fan color. 

I swapped in a new fan clutch this morning and topped off the coolant again, but it slowly climbed up to over 3/4 on the gauge. This time I had the front end raised a good bit.

After it cooled down, I stuck a meat thermometer in the radiator neck and started it up. When I think the thermostat opened, it started spitting/splashing coolant out again. The thermometer showed about 140, but was still slowly climbing.

Would an obstruction in the radiator cause this?

 

Could be.  The top hose supplies the top "tank" of the radiator, from there the coolant has to flow through many small tubes to the bottom  tank.  The small tubes are what get clogged.  Looks like you might have went past well-enough.  Might not even be the thermostat opening, but if you wanted to test, you could remove the thermostat.  Then you'll get immediate problems.

On 10/22/2017 at 1:17 PM, jeremy93ls said:

Pulled the radiator out, flushed the not-so-cruddy mixture out of it, then poured in a gallon of water with a measured bit of muratic acid and shook/rotated/flipped/worked the mixture throughout the radiator to hopefully clean the passages.

 

When it is hot, what happens when you squeeze the upper and lower radiator hoses(cap on)?

Here's another easy test of the radiator.  Disconnect the hoses and fill the radiator from the top with a garden hose.  You shouldn't be able to fill it faster than it runs out the bottom.  If it overflows you'll know the cooling tubes are clogged.