If it was low on fluid it can cause the transmission to slip, but it doesn't usually happen out of the blue at cruising speeds. (in my experience)
There aren't any aspects of your transmission that are vacuum activated.
#1 How low was it?
#2 Was it running hot?
#3 Does it have an external transmission cooler other than the cooling in the transmission cooler in the radiator? (almost confusing double-talk)
#4 Is there any leakage where the back of the motor stops and the transmission begins?
P.S. Make sure you have sufficient coolant in the radiator and the expansion tank (overflow/reservoir)
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Last edited by Warlock III; 09-22-2013 at 12:47 PM.