John Deere 4020 Powershift Transmission Filter Seal

I have a 1964 4020 with the powershift transmission, the transmission has two filters, and I have been having trouble with the seal on the forward of the two filters. the seal will all of a sudden start leaking,running a stream of oil out on the ground without warning, when removing the filter cover, it appears that the seal has actually been pulled in towards the filter like there may be suction being created in this area.
When you shut the tractor off, it also sounds like there may be a suction or pressure sound for a short time coming from the fill cap under the seat for the transmission, not sure if that is normal or not.
I have changed filters,seals,oil,checked screen in transmission,cleaned fill cap. Filter cap and sealing area seem to be good and everything straight and true. Transmission and pto seem to work fine. just need to try and figure this out as it happens without warning, and if using it in field or mowing, you may not notice it until you are low on oil.
I have had it to JD mechanic a couple of times for other items and had them look at it, but they have not come up with an answer yet either.
Any ideas what might be causing it?

Thanks,

1. Do you have the correct rubber "washer" for your tractor... the part # changes @ serial #91,000.

Older = #R39596, newer = # R34733

I'm GUESSING you MAY have the wrong one, and it fitting in the wrong "step" or "recess", either on the cover, or on the opening in the case.


2. (I'm not even sure this COULD cause such a problem), but have you examined the sump screen and cleaned it, if needed?

Packing is larger than bore. Are you using a new one? It should fit tight in the case. Be sure your cover is not cocked as you tighten.
Packing will not "suck in"
Be sure your bolt oring is not leaking.

Sealing washer is the correct one for serial #, and seems to fit proper, went through that process the first time it happened, thinking we had the wrong one.
Always make sure cap is on right and pulls in straight and seats good.
Always have to put in new washer when this happens, once the washer has been out of place it will not seal again.
Have drained oil and checked sump screen a couple times since this started, screen is always clean.
Had no trouble with it this winter or spring, used it for loader work and running auger. Put on the mower to mow ditches,ran it about half a day on the mower no leaks, pulled in front of shed to do some work on the mower no leaks coming into yard, started the tractor up to turn it around and started running a stream from the filter, left trail for about six feet, shut the tractor off and leak stops, put drain pan down, loosened up cap and removed it, and could see that gasket had been pulled in towards the filter, put in new gasket and mowed ditches for 4 days and has not leaked since, this is how it seems to go, just never know when it is going to decide to let go.
We have owned this tractor for 30+ years, and never had this issue with it until probably in the last 4 or 5 years, been trying to figure it out, but when it happens so randomly it is hard to pinpoint what the cause might be.

Wonder if the fill cap isn't doing it's job as a breather properly. Maybe see if air can be pushed through it? Or loosen it after working so the cooling of the air inside the case doesn't create a vacuum?? good luck