Yeah. Trannys aren't supposed to leak, period. They're a hydrolic system that's pressurized. Therefore, if you add too much fluid it will make your leaks worse, because the system will force it out any way it can. Usually it breaks the front or rear seal, but sometimes it breaks the pan gasket. Rarely it will force its way out of the governor, the modulator (blow the baffle), or the cooling lines (which would be BAD). That's how some people end up with coolant in their trannies, and ATF in their radiators.
Overfilling can sometimes be just as bad as underfilling. As
longhornmail told you it could cause clutch damage as well. You could end up varnishing your friction plates just as if you ran it low on fluid!
Pull that thing and do a seal job. Put it in, and test-drive it for a few days. Get those leaks stopped, and pay attention to the shifting. I would rather do a seal job than use that Gunk crap, and continue worrying about leaks.
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'69 3/4 ton C20 2wd w/ 350ci/400THM and a wood bed!
'69 3/4 ton Custom 20 2wd w/ ORIGINAL 350ci/4sp Manual and a wood bed (parts beast).
Seattle, WA.
*See pics of my trucks and project at
www.webshots.com!