Hydraulic problem in old 1737 Case Uniloader --HELP

I've been repairing a 1737 Uniloader from the early '70's, during the last year. I Completely drained the hyd.system replacing the dirty fluid with new. Instead of using ATF I opted to fill with a universal tractor hydraulic/transmission fluid simply because of cost, I got a good price on it and the fact that I use this fluid in my tractors all the time without any problems that I'm aware of. My mechanical repair was all good with the exception of the hyd. system. It is very slow to operate at all, especially when cold as it is now. The fluid is foamy and I've bled lots of air out. The system sat entirely empty for quite a long time and I wonder if something crawled inside one of the lines or maybe the filter or a major line is somehow blocked (dead mouse maybe). The system operated fine before being drained, and now hardly at all - struggles to lift the arms even 6 inches. Is this a fluid issue? Is the viscosity or consistency so far off that it won't work? Perhaps the pump isn't compatible with the fluid or could the pump go bad while sitting so long? Anybody have any ideas as to what should be the next step in solving this problem. Your advice is much appreciated. Thanks in advance for your thoughts. John

Don't think the change of fluid is the problem.

The foam is an indication of air being sucked into the intake of the pump. Typical causes of this are low oil level, a restricted suction strainer, air leak in the suction side of the pump plumbing.

Hydraulic systems are self purging of air if working properly, so no amount of bleeding will solve the problem. Now, if there is a suction leak, the longer the system runs, the worst the air problem will get. Each time the oil passes by the leak, the more air will be whipped into the stream. But once the pump is stopped, the air will rise up and out of the oil in the tank.

An old 1737 owner here, who messed around for a couple years trying to find why the hydraulics were so noisy and slow when cold.

Finally found it. the aluminum hyd filter base gets loose on the pump fitting and sucks air at the threads. The filter does not appear loose because it has a mounting bracket below the filter base that holds it rigidly in place in spite of the pipe inlet fitting being very loose.

Remove the filter can and cartridge, then remove the little support bracket under the filter base and remove the filter inlet hose.
Using a large Water pump pliars or pipe wrench, I about guarntee you will be able to tighten that filter base at least 1 turn.
Put it all back together and your hydraulic troubles will be over.

I have also had them strip out the internal hyd pump drive spline in the gear box shaft.
When that goes, you lose all hydraulics instantly.

Incredible, I can't thank you enough. I'd never have had the patience to find this obscure issue and I'm sure that from your response that I'll be able to get her going tomorrow afternoon. If I can ever return the favor, don't hesitate to ask. This is beyond my highest expectations and the reason that I think the world of the people that are out there willing to provide their knowledge for the benefit of helping others. You're the BEST! Thank You!