Big Block 727 Vent Leak

Hi Everyone,

I’m having a problem with a rebuilt 727 leaking fluid out the vent and would like to know if anyone has run into a similar problem. Here's the situation.

1972 Road Runner, stock 400, original 727. Jacked up on wooden blocks, completely level but high enough to slide the transmission out for the rebuild. Drained the torque converter as I was removing the flexplate bolts, used thread sealer on the plug before putting it back in.

Transmission had the Transgo shift kit TF-2 installed a few years ago, kept it during the rebuild. Rebuilt with a Transtar kit, all new clutches, bands (including reverse), etc. No modifications and kept the original torque converter.

Once the transmission was back from the rebuild, filled the torque converter with a quart and wiped Vaseline on the snout before sliding the converter into place. Put the transmission back in, connected everything, reused the original dipstick and tube. Put in six quarts through the dipstick tube, let it settle overnight.

Next, start the car in neutral (it’s still up on wooden blocks) and leave it warm up for approximately fifteen minutes. Check the dipstick, fluid showing just on the tip. Put half a quart in and that brings it to add pint mark. Add about a quarter of a quart and that brings it just shy of the full mark, about the thickness of two of the marks away from the full mark.

Car is still running, in neutral, so I go underneath to check the cooler lines, pan gasket, etc. Everything looks good then fluid starts dripping pretty heavily past the inspection cover. Remove that and maybe a pint in total comes out and still drips. Check the dipstick and it’s still showing just shy of the full mark.

Shut the car down and check underneath again. Began to think the converter hub seal was the leak, that I must have damaged it when sliding the converter in. Frustrated and it’s getting late, I leave it for another day.

Thought some more about it; asked my friend who did the rebuild and he suggested that I get some fluid out as the leak may be from overfilling and it’s not the converter seal but the vent.

Go back, start the car up in neutral, leave it warm up for a good ten minutes and check the dipstick. It’s past the full mark by about one and a half times the width of the space between the marks for full and add a pint. At this point, I haven’t added any more fluid than the last time which brought it to just shy of the full mark so I’m wondering why the difference.

Regardless, I have to get some of the fluid out. Disconnected the cooler feed line to the radiator, started the car back up, pumped out about half a quart, shut it down and reconnected the line.

Cleaned out the bell housing area completely, first with paper towels, then brake clean then more paper towels to make sure there was no remaining fluid.

Started the car back up in neutral, left it run for another ten minutes and there’s no leak. Checked the dipstick and it’s showing a full width below the add pint line, as though it’s down two pints. At this point, it looks like I’ve taken out three and a half pints per the stick but that’s not what I took out.

Added a pint, brought the level to the add mark and no leak. Added about half a pint, brought the level to just shy of the full mark (thought it would be in the middle between full and add) and the leak starts again. Check the dipstick again and still, just shy of the full mark.

I reused the original dipstick tube and dipstick. Mounted back in the same bell housing bolt due to the bracket welded on the tube so I’m sure it’s measuring the same as before taking the transmission out. The level used to be at the full mark and no leak before the rebuild.

I’m obviously overfilling it and that’s causing the leak but I don’t understand why the dipstick isn’t over the full mark.

When the car is in neutral, the tires are spinning forward at a decent rate. My friend explained that is normal. Could this be causing the dipstick to show it’s full but it’s actually overfilled?

Thanks in advance for any other ideas or tips to figure this out.

Mike.

put the parking brake on, if it's spinning that means the inernals are sloshing up fluid causing erratic readings

pump seal or pump o ring is leaking ,vent is too high for fluid to be pouring
out. The only time I've seen fluid come from vent is when trans is extremely hot or someone drilled a front drum for a brake and didn't move the vent to the rear of case.

I'd suspect the accuracy of the dipstick. The FULL mark on the stick should be about 1/4" above the ledge where the tube plugs into the case.

I read that it has the trans-go shift kit could it possibly happen to be a reverse manual valve body?

Hi,

Thanks for the tips so far.

Bill, I'm thinking the same thing and will put the car back on the floor once I've pumped out fluid to get it back down again, then go from there.

Jon, at this point I'm not ruling anything out but will look at everything again before having to pull the tranmission back out.

John, if I had replaced the tube and/or dipstick, I would be thinking the same thing but these are the original pieces. I could see if the measurement was off by half a pint at the most but to be off approximately one inch on the stick seems bizarre to me.

Henry, no reverse valve body, the Transgo kit was to firm up shifts.

Thanks again,

Mike.

I went to the rear vent setup and tapped and plugged the pump vent.