gMax 1.5+ glass bed level?

Just purchased a used gMax 1.5+ and the glass heated bed does not seem to be level from front to back. The front appears to be about 3-4 mm lower than the back. I have scoured the manuals and everything I can find but there doesn't appear to be any documentation on leveling front to back for a glass heated bed. Any tips would be greatly appreciated.
I had the same problem with both of my 1.5+ machines. It was a combination of problems. The two V slot rails Y rides on were not the correct distance from the outer main frame rails. This was putting the carriage wheels in a bind. That affected the level of the bed carriage as it moved front to back. The second problem was the washers between the Y V slot rails and the front and rear main frame rails were not all the same thickness. Correcting both of those problems got the bed close enough bed leveling could handle the rest.

Support gave me the distance the rails are supposed to be inset from the main frame rails. I can send that when I get home.