In case you didn't know a 630 s/n 12999 down the charge pressure is 80-90 psi , s/n 13000 up is 120-200 , ---------------when you checked the bronze case drain filter was there much junk in it , have you changed the hydraulic filter lately , not that it would help your problem but to cut it open and to see whats in it could give and idea what's up , I have just learned from the Ace at the local dealer than charge pressure can be lost in a bad drive pump ( sunstrand) , but the rotating group usually will put golden colored particles in the system when it wears , from what you say about heating up and dropping pressure sounds like a charge pump senirio
The gold particles are bronze from the swash plates that the rotating group runs on.
The groan you speak of, does increasing the RPM fix this? i found at low RMP my 731 would make quite a bit of noise under load but kick the revs up to almost full and it would hardly make a noise (if you could hear over the racket the engine made).
If you crack a case drain line you will get a fair amount of fluid out of the hose. You would really need to plug the line.
The only thing i can really recommend is to watch the pump temperature. If it starts cavetating from a blocked filter it will groan and squeel and the pump will heat up, and it will heat up FAST. When my suction line filter was blocked (and blocked it was!!!!) the pump was un-touchable within minutes. My filter looked like it was never changed or cleaned, there were a few alloy particles and a few bits of silicone stuck in there.
As you said, its almost 30 years old. Its done pretty well, just as mind did. Its still running strong and its new owner absolutley loves it.
If it still runs strong and doesn't heat up i'd leave it all alone. Just watch your pump temperature and you should be pretty well rite.
As Fishfiles said, checking the insides of the filters is a pretty good idea. It will let you know if there is a part failing or not. I have done it a few times before and was happy that there were no visible metal particles.