i have a 65 727, which has a internal seal on the output shaft that the front yoke slides onto. i believe its purpose is to seal the yoke splines from tranny fluid. the factory method for lubing the yoke splines was to add multi-mileage grease to the inside of the yoke. the factory yoke had a small hole in the rear of yoke to allow pressure build-up inside the yoke to escape. after 66 the seal was eliminated and tranny fluid is allowed to lubricate the yoke splines. question-my yoke is the later version & doesnt have the vent hole.i know my yoke fits the seal good ,i have to lube the inside of yoke get it to slide onto the seal. at about a 10 mph roll on rough road i noticed a sort of bouncy feeling in the rear of car. could pressure inside yoke be hindering rear axle movement? if so, should i drill vent hole in yoke or eliminate internal seal. much easier to drill vent & internal seal may help isolate vibrations . what do you guys think?

Drill a hole and you have instant transmission fluid leak. Leave the solid plug in the yoke.

the internal seal is designed to seal the outside diameter of output shaft to inside diameter of yoke. the vent hole in yoke was factory drilled. i dont see how it could leak trans. fluid.

the internal seal i refered to is about 3-4 inches in from end of output shaft. it seals the inside of the front of the yoke to the output shaft,therefore no trans. fluid gets into the yoke,factory used grease for yoke splines.it was only used on 65 and early 66 model slip yokes. so, if yoke is sealed at front and has no vent hole at rear its possible to create hydralic pressure with movement of rear suspension.

The output shaft with the yoke seal (at right in the pic below) needs to be used with a matching slip yoke that has the correct ID and length of missing splines inside the yoke and the vent hole is necessary or you can get air hammering which can account for the "bouncy feeling".

The yoke seal was also used on '66 and possibly some '67's.

thanks john. i knew if you saw this post you would know what i was talking about. so, what would you do? i know my yoke fits the seal( i can feel it engage). should i drill vent hole in yoke(size/location) or remove the seal? thanks

Sorry reread and see you are working on an older transmission.
Not on later ones though.

You can drill a small hole in the yoke end plug but if the internal seal isn't a perfect seal it will weep fluid. If it sealed before the yoke change it should be OK.

The original hole is dead center in the plug, don't know the size but .060" should do it.

THANKS AGAIN JOHN. I THINK I"LL TRY THE VENT HOLE FIRST. I'VE HAD THE DRIVESHAFT OUT SEVERAL TIMES & NEVER HAD FLUID IN THE YOKE. I SUPPOSE I COULD PLUG THE HOLE & REMOVE THE SEAL IF IT DEVELOPS A LEAK. I HAVE HAD THE TRANNY OUT & APART SO MANY TIMES I QUIT COUNTING.FAINLLY(WITH HELP FROM THIS BOARD) GOT IT ALL SORTED OUT & JUST DIDNT WANT TO OPEN HER UP AGAIN.BUT I HAVE GREAT (HOW NOT TO ) EXPERIENCE NOW.

Now that I've read this thread a few times, my question is about the reverse situation. I replaced the original trans with a 1976 unit, but used the original yoke. Should I tap & plug the vent hole? Out of all my spare yokes, the only one I've got that's the same length has a vent hole in it too. Or is the added lenth only for the internal seal?

not sure about the length, but it's easy enough to plug it(tap & plug) or plug weld it,you may not have to disassemble it to weld it,not sure.it most likely will leak if it's not plugged.i believe you could look inside one of your other yokes & measure how far in the splines begin from the front, then check this measurement in comparison to same measurement of vented yoke. if difference in the two matches difference in total length, you would have exactly the same engagement(yoke/splines). hope this wasnt confusing.

Makes perfectly good sense. Glad you started this post,as my original '65 727 didn't have the shaft to yoke seal when I rebuilt it. I would have overlooked the whole thing.