So i have a 68 impala wagon with a 307. The radiator hasn’t been flushed in at LEAST 12 years. The vehicle blew a radiator hose and i got a look at the radiator fluid and i was like, oh dear lord, this is bad. I would refer to the color as “burnt umber mixed with blood”. I believe no oil is in the water thankfully. So i drained the radiator and flushed it several times and the liquid was no better. if u filled a 5 gallon bucket and let it sit a few hours there would be like a half inch or more of sediment. so i realized it was time to get serious and took off thermostat and disconnected hose at radiator and flushed block. then filled it back up, drove it and flushed it again. still dark dark red/brown. then i just started car with thermostat back in it, and would let it heat up, thermostat pops and gush’s gunk and i kept filling radiator and thermostat would close and reopen and gush. i did that like 10 times. No real change. then i went to O’reillys and got a radiator flush liquid. That did seem to help, it turned liquid almost black and tons more gunk came out. then i restarted cycle and flushed block like 15 more times like before, just standing there dumping water in radiator as thermostat popped over and over and over filling bucket after bucket. now its STILL dark red. but it at least looks a little clearer as it comes out. but when u look in bucket it is still fully opaque. Short question LONG, how long should i keep this up before i just put in some anti freeze and call it good? it seems like its never going to end. its insane, ive never seen anything like it. car runs like a dream when u drive it though.
You need to remove the engine coolant drain plug and then flush the cooling system.
What you’re seeing is the rust sediment in the engine block, and removing the drain plug will help get rid of some of it.
Tester
My cars are 30 and 50 years old. If I had that problem I’d continue w/the flushing until it came out 100% clear water. I’ve always used the same flushing method as you presumably do, removing the top hose to the radiator, running the engine (with thermostat installed) and letting the water pump do the flushing for me, keeping the radiator topped off with a garden hose. I always capture what comes out of the top hose in big containers (garbage can usually) , and take it to a coolant-recycling place. (Note, not recommending this method btw, as the cold water from the garden hose could conceivably pour out the bottom of the radiator, go into the engine, & crack the hot engine block, although never had the problem myself. )
Suggest going forward to flush and replace the coolant every one to two years. It is possible serious internal damage to the engine has already been done.
thank you for the info.
can u guide me to that a little more? is that a pic of a 307? i can’t tell where under the hood u are. presumably because your car is all nice and clean and not black with the grime of ages. 
The drain plug is on the side of the engine block above the oil pan.
Here’s an example:
It may be a square head plug:
It may be a hex head plug:
It may be a recess hex socket plug:
Or, it may be a pet cock valve.
But somewhere on the engine block down by the oil pan, there’s a means of draining the coolant from the engine block.
Tester
Heh heh, another reason we changed coolant every fall back in 1968. Not to bring that up again. Hopefully those plugs are not rusted solid like mine seemed to be. Like I said I made an adaptor that screwed on to the garden hose and fit the radiator hose to be able to power flush the engine. Just copied what the guy at a radiator shop used.
If you are uneasy about the block plugs, you could always remove the water pump and shove a garden hose in one hole and let it flow for a long while, then swap holes and keep doing that until the water comes out clean… You need some good water pressure and volume to do it… And not live in California… lol
All smallblock Chevys for 50 years have block drains on each side midway along the block just above the oil pan rail. If they’ve never been removed before, they will not come out short of a nuclear explosion.
I’d flush through the heater hoses. But that sediment in the block is going to stay there.
Yah…now that u mention it… maybe its not a good idea to remove that. dang. if that breaks, im SOL.
It will either come loose or it won’t. I don’t think any in between since likely an iron plug on a 68. Getting access to it might be hard without ramps. Just never did it myself. Maybe pb blaster fir a couple days might help.
“flushed block like 15 more times like before, just standing there dumping water in radiator as thermostat popped over and over and over filling bucket after bucket. now it’s STILL dark red.”
I am wondering if you have a leak in one of the tranny fluid tubes in your radiator causing tranny fluid to mix with your coolant.
Well I guess at this point you should just replace the radiator. Unfasten the top and lower hoses and flush just the block and see what happens. Pull the thermostat out for full flow. You can do a pressure test on the trans hoses in the radiator to see if it was leaking. I don’t even know if they did that in 68. Can’t remember. If so then you will want to do an immediate transmission fluid exchange.
I wouldn’t even try removing the block drain plug myself as a diy’er. If OP wants it removed, suggest to hire that job out to a pro. Depending on the cooling system configuration, it may be possible to get a pretty good block flushing by removing the lower radiator hose. Removing the radiator and turning it upside down for a reverse flush makes sense. An old radiator might develop a big leak from being disturbed like that, but better to know now than later.
Engine builders will tell you it is at best a 50/50 proposition the factory steel plugs will come out IF the engine is out of the car, dry and heated with an oxy-acytelene torch. Having rebuilt a few engines, I can confirm this is correct.
Oh, they’ll come out. The question is, will you be able to thread in a new one afterwards? 
Not a big deal to drill and tap a slightly larger hole there, the bosses were pretty beefy in comparison. Bought that tshirt. But more stuff will probably have to come off to have that kind of access.
1968 Imp w/307? Betting I could reach them with the engine in the car. Pretty fair amount of space IIRC. Assuming stock of course.
Rust sediment doesn’t fit the description of the color to me. I think @weekend-warrior might be onto something. Easy enough to check…
Yah…now that u mention it… maybe its not a good idea to remove that. dang. if that breaks, im SOL.
Hello thanks for your info. If that was the case though, wouldn’t i be running low on tranny fluid? my tranny fluid is still full and hasn’t chnaged in awhile. it does have a small leak, but its from a seal.
if this was the case wouldn’t i be low on tranny fluid? im not sure how to test whether or not this is happening.
Replace radiator and remove all doubt. Replace trans fluid too. You know you should anyway.