Need a little help with 727 output shaft leaking........
 

I posted aout this problem years ago, but here goes again. My truck output shaft leaks after it has been parked for a couple of days. It isn't leaking at the seal. The fluid is migrating down the splines and leaking out the open back of the slip yoke. I can't imagine what Dodge was thinking when they designed this with an open ended yoke. I am assuming that fluid is draining back from the TC over a period of time and causing the leak (hard to figure though since there is no noticable delay in shifting after startup). I tried pulling the yoke and applying sealant. That worked untill I drove the truck a few hundred miles and then it was back to leaking. Anyone ever fixed this problem? Does anyone sell a replacement yoke that might have a plug for the slip yoke that could be installed after the yoke nut? Does the TC have a valve that could be leaking back into the pan or is the fluid coming back through the transmission valve. I don't want to change the valve and still have the leak. I wonder if it would work to fill the end of the yoke with sealant after the nut was tightened. Would that mess up the trans (I know that one spline is left off of some 727 yokes for venting but I don't know if that is the case with this yoke)? The leak sucks but there appears to be nothing wrong with the trans. Thats why I have just lived with the leak for all this time but I would like to fix it now.

I have no theories as to why it is doing it but I would say that unless you got stupid with the sealer that you won't hurt anything.

I'm confused. How about some information on exactly what your truck is and the driveshaft configuration.

Sorry about that. For some reason my signature has disappeared. That's where my truck info was. It's an 89 D250 Cummins. EDIT: now with this post it's back. Drive shaft is 2 piece with swing.

So your truck has the bolt on yoke at the trans and not a slip yoke?

All 727s have a vent at the pump but if you are worried about venting you could drill a small hole in the top of the extension housing, thread it with a pipe thread tap and install a differential vent with a cap.

Is the nut that holds the yoke flanged or does it use a washer?

Sorry for the late responce. The yoke is open. If I remember correctly, it has a raised shoulder on the nut. The nut tightens on a raised shoulder of the shaft (not against the yoke). The yoke has the ability to slide in and out on the shaft just as a closed yoke would. I am assuming that the fluid should never get to a level where it could leak from the splines.

Reviving my old thread. Still got the same problem. Truck sits for 2 or 3 days and starts leaking from the splines of the output shaft. It has to be coming from converter drain back. I don't have a delay in shifting but the previous owner may have had a kit placed in the VB to fill the converter on startup. I do have a noisy converter until I shift through the gears sometimes. I think it is probably air in the converter. Did the trans in my 89 come from the factory with a cooler line check valve? Where can I order a replacement check valve? A google search for A727 drain back shows that it is a common problem for this trans and the fix is to change seals until it stops leaking. That's fine on a trans with a closed yoke, but is useless on mine with an open yoke. My only option is to stop the trans from draining back and overflowing the yoke splines. Any ideas on repairing this? Silicon worked for about a week, but one extended run with the truck tore it to shreds. Remember that I said previously that the nut and washer tightens against a raised portion of the output shaft and the yoke has a slight movement fore and aft just like a closed yoke would. I sure hope someone has a suggestion to remedy this problem.

Any reason you can't run a closed yoke form another tranny? The A518 should be the same spline I believe. Might have to get a new primary driveshaft made though?

Thanks thrashingcows. That sounds like a good idea to swap the yokes. Anyone have an idea if they will actually swap out? Anyone got a spare for sale?

Well I've been on google most of the day trying to find a check valve for this monster. Only valve I can find is a 5/16 valve. My cooler lines are 1/2". I have about decided to put a 1/2" ball valve in the rubber portion of the line coming from the cooler (the cooler is at the top front of the radiator with the evaporator below that). I can just close the ball valve when I am not planning on using the truck for a while. This would serve 2 purposes. First it would let me know if a valve would correct the leak and second there would be no chance of it plugging. I really would like to know what the wear is that allowed the drain back in the first place. I don't assume it did it since new and the trans works perfectly (it appears to have been rebuilt not long before I bought it in 2005) and never had a sign of a leak until I retired and the truck started sitting for a week or more at a time.

Converter drain back was standard equipment from the factory, no extra charge. I don't ever recall seeing a bolt on yoke that still had fore and aft movement on the output shaft. Something isn't right there. The yoke could possibly be off an older 727 from a crew cab or something. There is no reason for it to have movement, something isn't right.

Take a picture of what you have as I agree with angry johny something doesn't sound right with what you have.

I'll try to get a pic tomorrow after this storm leaves the area. It won't show much except a nut and yoke. The trans would have been swapped along with the yoke or else the output shaft would not have had a threaded end for the yoke to attach. Since I have one of the first Dodges with a Cummins produced, I doubt that happened. The yoke doesn't have a large amount of movement but it does have enough to allow the driveshaft to flex without breaking anything. I can't believe no one else with an 89 has chimed in. My stainless steel ball valve arrived today from Amazon so tomorrow I will install it.

The only place I have seen what you describe is on some of the cummins powered military tug trucks that have a midget rear drive shaft. I have not seen it on a cummins pickup truck ever.

Yea JD. I spent 20 years with those tugs in munitions as a technition with AFRES. Same Cummins, same trans (I think),and a rear end geared so low it could move 15 tons like it was a cardboard box. Top speed of about 30 mph because of that rear end. Reminds me of my truck except top end is about 70. I'm going to look into getting that rear end regeared