A Class Motorhome/Allison question
 

Hi Folks,
Fixing up an older Rockwood 30' motorhome with early 5.9/Allison 4 speed....changing the trans fluid today and wondering about the reservoir under hood (looks like a windshield washer container). While draining fluid from sump, I thought that this would also drain, but no. Is there some pressure issue here, should the hoses be undone down low to remove old fluid? Procedure for getting as much old fluid out would be great,
Thanks,
Seann