If you have a leak, I think that the cooling system sealants can make things worse in the long run.
My anecdote: My 97 Ford Thunderbird DD came with a composite (i.e. plastic) intake manifold through which hot water circulated to the radiator thermostat in the manifold. It cracked and leaked and Ford replaced it under warranty. They replaced it with a duplicate manifold, which failed outside of the warranty period. It was a hair line crack. The Ford dealer wanted a little over $1000 to replace the manifold (again). I switched over to pure 100% ethylene glycol. It boils at about 360 degrees F so with ethylene, my coolant system didn't become pressurized. Not being pressurized, leakage was minimal. I drove the car like this for a year. I probably put in a gallon of pure antifreeze every three weeks ($15 a gallon). Finally, after watching YouTube videos about replacing the intake manifold, I bought one for $320 and replaced the manifold myself...piece of cake. My 68 Corvette, my 94 Cadillac, and my 97 Thunderbird don't have a drop of water in their cooling system.....100% ethylene glycol. My 68 has been water free since about 1972/3. It still has the factory heater core. In 1972/73 the 68's factory aluminum radiator had rotted out. I bought a new OEM and it's corrosion free after all these years, although for my ZZ4 I replaced it with an aluminum Be Cool BB radiator. When I removed my factory 68 engine in 2006 (?) I looked into the water chambers.......absolutely no rust...only a thin film of pure white silicate deposits.
The first time I drove my ZZ4 powered 68, I just had the upper engine water coolant hoses pushed on hand tight...no hose clamps. Knowing I wasn't going to pressurize the cooling system with pure glycol, I didn't see any problem. Opps, the water pump can create coolant pressure..forgot that. On my first test drive around the block, the water pump blew off my heater hoses and sprayed the engine compartment with glycol. It saturated the fiberglass firewall and transmission tunnel blankets. Had to replace them. Note: Only the 68 and 69 Corvettes have this really NEEDED cooling insulation.
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If you think using pure ethylene glycol as an engine coolant is something bizarre, treat yourself to a long evening watching YouTube videos of WWII Mustangs, Spitfires, Mosquitoes, etc in action. The fastest internal combustion reciprocating engine vehicles ever made. They all used pure ethylene glycol..no water. Rolls Royce pioneered the use of ethylene glycol as an engine coolant for their Merlin engine. Several of the WWII fighter planes could do 450 mph on pure ethylene glycol coolant. Works for me at highway speeds.
