1997 Disco 1. Took it on a trip over the mountains and it overheated both ways and started dumping coolant out of the black reservoir through the seam (the reservoir is made of 2 pieces, top and bottom). I replaced the thermostat and put 3 gallons back in. When i started her up, i let her run for a few mins and it starts spewing coolant again. any thoughts? also, how in the hell are you suppose to know if you overfilled. there doesn't seem to be a max/min line and again, the reservoir is solid black plastic so you cant really see it sits

 

It's a common occurrence. The coolant tank seam gives out unexpectedly and makes a hell of a green mess. Mine gave out last year in a grocery store parking lot.

This is the replacement I use which is translucent so you can see the coolant level. Plan on replacing this every 3 years or so. With this tank I fill until about a half inch above the seam line.

 

+1 translucent reservoir. Mine is All-Makes. Maybe run the system unpressurized, then the reservoir will last "forever", but need to judge carefully at high elevations, maybe use 60/40 concentration. Might be possible to get lower pressure rated cap (e.g. 8 vs 16 psi)?

 

Did you turn your heater on? And do you have an UltraGauge to monitor temp?
After replacing ALL the cooling system components, I found my issue was the fan clutch was bad. Of course it was the last item on the list I replaced.
Good Luck,
dcphotos

 

Without totally draining the system 3 gal is too much. You might use 3 gal if it is totally empty.

If you look into the reservoir before it is filled you will see a vertical piece of plastic in the middle of the tank in the form of an ‘x’. When that is covered the truck is filled.

If the seam of the tank is split it is gonna leak no matter what.

 

Also make sure you have a solid fitting cap on the reservoir. This was my issue years ago and it was the cheapest but last thing I replaced after everything else related to the cooling system.