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Ok
I have a 64 1/2 mustang that I recently purchased; my dream car. I know the early Mustangs did not have an overflow reservoir for the radiator. They just had a hose that ran towards the ground.

I noticed a few times this winter that when I let it run for a bit, antifreeze would drip to the garage floor. Today I was working on the car (drum brake rebuild). Afterwards, I topped off the radiator, which was about 3 inches low, before filling (the car engine was cold for reference). I started the car and let it run. After about 10-15 minutes a fair amount of antifreeze was on the garage floor (maybe a cup or 2?). Is that normal or should I be looking at potential issues?
 

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Current cars include 1969 Mach 1 and 1970 Cougar XR7 convertible
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Congrats on your recent purchase and welcome to the forum.

"Topping Off" the radiator may be the issue. A full radiator coolant level is slightly above to top of the core. In other words, during operation the coolant expands and fills the air space in the upper radiator tank. If over filled with coolant it will self level. Beyond that, virtually every component of the cooling system can be the source of radiator puking. Air flow through the radiator as well as coolant flow through the engine block are critical to keeping the engine running in its operating temperature range. If the car appears to be overheating, start with measuring the coolant temp with a known accurate gauge to calibrate the temp gauge in the instrument cluster. Sometimes the engine is fine and the temperature sending unit is the culprit. Lots of things to look at. Avoid starting any diagnosis by randomly loading up the parts cannon and firing it at the car! It can get costly fast and may not make a difference.
 

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You over filled it. It should be filled to just cover the fins in the radiator. About 1" below the neck. This allows room for expansion.
 
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I see this as your first post, Therefore, "Welcome" to a great resource for your Mustang questions?
In my opinion, the installation of a coolant recovery system has two primary benefits.
1. Reduces contamination from poisonous coolant on the ground and in your garage wherein "Fido" may cruise.
2. A recovery system expels air that gets trapped in the block and replaces with coolant. Coolant absorbs heat, air does not.
The coolant recovery system with the correct radiator cap, will allow the coolant to circulate through the engine as designed. When the car is parked and the engine begins to cool, a negative vacuum is created in the radiator, pulling coolant back into the "Rad". Over several heating and cooling cycles, this process evacuates air from the cooling system. Your course of action, is to insure the coolant tank is kept filled to its recommended fill level. A some point, depending on your driving intervals, when the cooling system is cool and can, safely, remove the cap, the coolant level should be at the very top of the Rad where the cap seal meets the Rad seal lip. Thereafter, watching the level of the recovery tank will provide an indication of any impending coolant leaks. BTW, I too have an early Mustang assembled in May 1964 and do run a recovery system. Mine is the cylindrical canister near the washer fluid bag on the right with the aluminum cap. Typical it would be mounted at the RAD but our horns are mounted there on the frame , therefore, had to move to this location.
Hood Automotive fuel system Motor vehicle Automotive design Car
 

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A recovery system expels air that gets trapped in the block and replaces with coolant. Coolant absorbs heat, air does not.
The coolant recovery system with the correct radiator cap, will allow the coolant to circulate through the engine as designed. When the car is parked and the engine begins to cool, a negative vacuum is created in the radiator, pulling coolant back into the "Rad". Over several heating and cooling cycles, this process evacuates air from the cooling system. Your course of action, is to insure the coolant tank is kept filled to its recommended fill level. A some point, depending on your driving intervals, when the cooling system is cool and can, safely, remove the cap, the coolant level should be at the very top of the Rad where the cap seal meets the Rad seal lip.
Exactly this. One thing I would ad is make sure that whatever connection you have from the radiator to the overflow connects at the lowest point of the overflow/expansion tank, or there is a tube inside going down to the bottom of the expansion tank. Otherwise you risk sucking air back in. I have my battery relocated so that provides a great spot (also my cap is on that side). Here's a shot of my setup for ideas. The tank is just a simple overflow tank off of amazon, all the ports are ORB style, so it was easy to find/make fittings to accept the NPT push to connect fittings. I also have a "filter" fitting you can't see on the top of the tank to allow for it to breathe and to do its job.

790647
 

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I like the above pic ...... but in a pinch while you are waiting for a nice looking tank - I did this >>>> it actually worked, sucked up when cooled down and pushed out when hot. As the above post states - it has to be lower than the top of the rad.

790648
 

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I like the above pic ...... but in a pinch while you are waiting for a nice looking tank - I did this >>>> it actually worked, sucked up when cooled down and pushed out when hot. As the above post states - it has to be lower than the top of the rad.
Our preferred overflow when i was in FSAE were nalgene bottles. The way the rules were worded, a 1L nalgene was perfect. Those things were damn near indestructible and held up just fine against the hot engine temps (we used one for crank breather too). Also cheap and easy to replace if there was a rupture, so we "designed" for the eventuality of one getting smashed by a cone 🤣 Moral is if its dumb and it works, is it really dumb?
 

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I see this as your first post, Therefore, "Welcome" to a great resource for your Mustang questions?
In my opinion, the installation of a coolant recovery system has two primary benefits.
1. Reduces contamination from poisonous coolant on the ground and in your garage wherein "Fido" may cruise.
2. A recovery system expels air that gets trapped in the block and replaces with coolant. Coolant absorbs heat, air does not.
The coolant recovery system with the correct radiator cap, will allow the coolant to circulate through the engine as designed. When the car is parked and the engine begins to cool, a negative vacuum is created in the radiator, pulling coolant back into the "Rad". Over several heating and cooling cycles, this process evacuates air from the cooling system. Your course of action, is to insure the coolant tank is kept filled to its recommended fill level. A some point, depending on your driving intervals, when the cooling system is cool and can, safely, remove the cap, the coolant level should be at the very top of the Rad where the cap seal meets the Rad seal lip. Thereafter, watching the level of the recovery tank will provide an indication of any impending coolant leaks. BTW, I too have an early Mustang assembled in May 1964 and do run a recovery system. Mine is the cylindrical canister near the washer fluid bag on the right with the aluminum cap. Typical it would be mounted at the RAD but our horns are mounted there on the frame , therefore, had to move to this location.
View attachment 790639
Do you use the standard radiator cap or is a different one needed to allow the fluid to return back to the radiator from the overflow tank?
 

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I added a small overflow tank on my 65. Keeps it off the garage floor.
 

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Do you use the standard radiator cap or is a different one needed to allow the fluid to return back to the radiator from the overflow tank?
The standard cap with a plunger and spring is fine. This was a bit counter-intuitive to me at first but after a little thinking it makes sense. You'd assume with the spring and rubber gasket-like thing on it that any "Reverse flow" would be near impossible. Stuff (fluids, the funds in my wallet, gasses) always want to from the "highest pressure" to the "lowest pressure". Its a science thing. So even with that rubber gasket on the plunger under the cap, when the coolant is coming back down to a colder temperature, it will contract and create a vacuum in the cooling system thats actually at a lower pressure than ambient air pressure. Once it reaches that certain point it'll go SHLORP and suck the coolant back in.

With the above mentioned nalgene bottle it was actually pretty awesome to see that happen real-time.
 

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Do you use the standard radiator cap or is a different one needed to allow the fluid to return back to the radiator from the overflow tank?
There is a cap designed for this purpose. I think, it's called a double sealed closed cap? It's been too long and I don't have my car available to pull the cap for reference. It's been discussed here a few years ago. Search?
 

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There is a cap designed for this purpose. I think, it's called a double sealed closed cap? It's been too long and I don't have my car available to pull the cap for reference. It's been discussed here a few years ago. Search?
It s a cap with two rubber seals, the top seal is needed to create a vacuum.

Here's where I show my age, I thought both of you were crazy. I've only ever know the "recovery" style radiator cap.

Picture for 1000 words (shows both the main seal and the "Additional rubber seal":
790655
 

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If this becomes a radiator overflow pic thread mine always gets a few comments..
790665

All the aforementioned commenters saying your coolant level is too high are correct. When I first started driving my stang I was confused by this same occurrence until I found and read a pdf of the original owners manual that said just to cover the fins. Adding the overflow can kept my engine bay a little cleaner until then
 

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Before all the fancy plastic bottles and aluminum tanks was the beer/pop can ! If yours is leaking at idle in the garage I would start with making sure (testing) the rad cap is good before worrying about adding a tank.
 
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