Toyota Repair: previa 95 all-trac S/C, viscous coupling, coolant

Question
Hi Ted,
I have a 95 Previa S/C all-Trac with 116K miles. From what I have researched online, it seems have a malfunctioning Viscous Coupling, it is not too bad actually, just occasionally happens after long and high speed trip. IS there a relatively low cost fix for this?
Another thing puzzles me is that it seems the cooling system sometimes can trap some air in there to cause overheat, this problem can fix itself overtime, but I do not understand how the air get trapped in there and how it gets out. Have you seen this before? thanks!

Answer
Can't help with the alltrac problem.
Normally the air can be removed from the system by running the engine at around 1800 rpm and adding coolant to the reservoir bottle as needed this may take about 5-10 minutes, there is no other way that I know of and is the factory procedure to purge the air out.