580ck No power in reverse

I am new and bought a 1970 580ck named Missy. She has done quite a lot of work in the last month and is having problems with no power in reverse. I do not think it is a pump or transmission problem, it feels like when shifting the shuttle to reverse there is no real engagement not like shifting into forward. If I am not careful I can lift the front wheels off the ground when the shuttle engages. I have a service and parts manual but do not know where to start. Does anyone have a clear picture of the testing ports on the shuttle control valve and the size of the fitting required to test the pressures. I can only find the clutch pressure test outlet with my service manual. Any help would be greatly appreciated. Thanks

580ck No power in reverse