case 580D Shuttle Leak

Last winter I bought a Case 580D. Seller said power shuttle has just been rebuilt. He was a very nice older gentleman and I believe it was. Here is the issue: when pushing into a pile of dirt, the shuttle leaks when the tractor begins to push under heavy load. After a bit the fluid level drops enough that it will not push at all. Fill it back up and all is good. It seemed to be leaking out of the vent tube. I have a service manual, but the pressure tests intimidated me and I thought I would just pull the power shuttle and examine it. The shuttle is out and the first thing I noticed is that the bolts that hold the cover on the valve body (right side of the shuttle just above the filler tube inlet) were loose. The top two were especially loose. Since the breather tube is just above that, I am wondering if hyd fluid could have been coming out of that loose valve body cover. I then started disassembling the shuttle to look for problems but everything looked good to me. I am by no means an expert at looking at the guts of a power shuttle. The clutch packs seemed fine, and there was hardly any debris in the oil pan or on the pump suction screen. The only other thing I found was that a thrust washer was missing. This is the thrust washer for the clutch hub behind the shuttle pump assembly. The surfaces on either side of where the thrust washer should have been looked fine with no signs of damage or scoring. Would the loose bolts on the valve cover cause a shuttle leak under load, and how would the missing thrust washer affect the shuttle performance? Thank you for any input.