looks like two possible leaks occuring at the same time. The reference to the leak stopping when you lower the blade seems like the pump drive seal is leaking under load and is getting worse with more work. The leak at the adapter plate could be the "o" ring leaking trans oil due to hardened seal and maybe extra hyd oil in the flywheel housing

Here is the flywheel drive end. Yes, we had a lengthy post on a similar project awhile ago. Believe the fellows name was Bill Holden, had a D6C direct drive that had been pretty well beat. Oil clutch was shot, along with pump drives and pumps. Included cracked bell housing and gouged out area behind idler gear. Guess he finally got it together........cause have not heard from him since. Was about a year ago I'd guess.

I like the overloaded scavenger pump explanation, either from leaking torque converter, weak scavenge pump or blown pump shaft seals but if it was hydraulic leakage into the bell housing the transmission oil level would increase along with the loss of hydraulic fluid but that does not seem to be the case.

Also note there are slight differences in the hydraulic pump flange oil ring seals and one has a recessed groove and others do not. May effect sealing if wrong one is installed.

I can confirm that I'm losing transmission oil and not HYD oil...

Maybe I was smoking something good when I thought the leaked stopped when I put the blade down...

I ran it today, and now I'm down to about 1 inch below the full line on the Trans dipstick. The sight glass on the HYD tank remains full...

I degreased the areas of the bell housing where both pumps attach. I can confirm I'm only leaking where the front O-Ring is for the adapter, no other leaks.

It's leaking pretty much all around that adapter plate.

I noticed while checking the Trans, how much pressure build up there was. The fill cap was hissing pretty good.

Is there supposed to be a vent for the bell housing? Could that be clogged?

What meters how much oil is taken to fill the bell housing from the Trans? This seems like too much oil from splashing... The oil level in the bell housing, how high should it be, up to this adapter plate?

Should I change this O-Ring and go from there?

Would my converter temp gage show any signs of a leaking converter? The gage always is well in the normal range...

As a matter of fact, the leak didn't start getting worse until I checked that trans fluid last week for the first time in quite a long while. I was fiddling with the cap, didn't know how the relief worked... Is it possible I tightend it up, making the trans build up more pressure?

The transmission filler cap has a breather on it....must be clogged.
The bell housing should have very little oil in it, just what leaks from the torque converter (they all leak some) which is returned to the torque converter by the scavenge pump. The pump is only rated for 3.2 gpm on the D6C. Windage is what drives some oil up into the hydraulic pump drive area. Yes, would seem to be time to go for the adapter flange/oil pump "O" ring replacement.
Torque converter temp would rise if leakage was bad enough.

I'll change the o-ring and go from there...

My HYD tank only has a drain plug, no valve to hook up a hose. Any suggested way to pump out the majority of the oil through the filler tube as opposed to wasting all that oil?

So the 2 seals I should buy are:

9M5894 & 1H6227?

I would have a 5F3106 "O" ring on hand also unless you can verify what "O" ring is correct for the 6J7400 pump. I believe it's the 1H6227 but I can't verify.
I use the cardboard or sheet metal trough method to get from drain to open bucket/s.....11-1/2 gallons when full.
Get new filter elements while your at it 4J816 (says it takes two)

Guess you could suction out some oil after you take the strainer out of the fill pipe.

Couple of things to check when you do the R&R:
Fit the adapter plate to the pump without the "O" ring to make sure it seats properly and will compress the "O" ring.
As edb mentioned check the bell housing bore for any signs of cracks as well as the adapter, also any cracks radiating from stud/bolt holes and buggered up threads.
Also check adapter for warpage in case it was tightened without proper seating.

Well, I went to the lot today, took off work

I put 5 gallons of oil into the tranny, and it still was below the full mark.

I decided I did not like the o-ring as the sole explanation for the loss of all this oil.

I decided instead of doing some more logging, the day would be spent finding the root cause of this leak.

I pulled off both belly pans, as oil was dripping from both of them. Of the 6 bolts holding the rear pan on, 3 were broken off.

The front pan had 4 of the 8 bolts already broken. I broke one more on the front one, now it only has 3 to hold it up.

When I finally got one of the bolts out of the front pan, about 2 gallons of water came out of that hole! No wonder so many of these damn bolts are busted, why didn't CAT put a drain hole in the damn frame rails!

Once the belly pans were both off, I could see on the left side of the machine when it was running, oil was predominantely coming from the steel HYD tubes right along the frame rail. I cleaned up all the crud, in the process I found this wedged against the lower steel tube:



It gouged a hole in the lowest tube:



Once I had the area all cleaned, I fired up the machine to confirm this was the only tube with a leak. I removed it.

While doing so, I noticed this mount plate for the tube was broken:



This further allowed that tube to vibrate and wear away, until it sprung that leak...

I welded up the plate:



And put a patch on the tube:



I'm not real fond of my weld job on that patch (it's mostly ground away in that pic), had a hard time finding a wire speed & heat setting on my MIG that was satesfactory... I have no way to pressure test it either... I may just buy a new tube if it does not cost too much... Anyone know how much pressure is in that line? It's item 32 on this drawing:



I guess I may as well get a new filter for the steering clutch & transmission lubrication system while I have that drained.

I'm still going to change the o-ring on that adapter as well.

I just knew that leaking o-ring could not be the cause of an oil leak of that magnitude...

Well, now we hear the rest of the story......certainly is different from previous version. You didn't say it was leaking like a sieve and pouring out of the belly pan. Better late than never on the sleuth work. Seems like you have repairs under control. I would have brazed the line patch. Pressure in that line is relief valve limited to around 350 psi.

Well, now we hear the rest of the story......certainly is different from previous version

There was so much crud in the belly pans, and around those lines, it did not leak like a sieve at first...

The belly pans were double the weight due to all the oil soaked dirt in there...

The hole in the line was right at the crosmember, so it leaked down both sides of it, dripping off both pans.

At first it was a few drops, by this morning it was a constant stream... I guess it took some hours to saturate all the dirt...

I don't have the equipment or knowledge on how to braze, that's why I used the MIG...

I should be good for 350 PSI as long as I didn't leave any pin holes or thin spots...

Guess I'll find out...

Thx again for all your help OM...