727 Reverse servo problem

I'm definatly no expert on atuo transmission so I'm a bit in the dark here.
Put a rebuilt 727 with reverse paten manual valve body, bolt in sprag, 4000 stall, from memory it has a 5.1 lever.
On the first start up I noticed it slipped in reverse so I re checked the fluid level and all was good.
Took it for a run and the box changes perfectly in all forward gears, nice and sharp, go's in to reverse perfect and on level ground rolls backwards put a bit of gas and it slips, if I try to back up a hill it justs slips and slowly go's backwards.
I have checked the cable on the shifter and all lines up spot on, running full mineral oil, trans cooler & deep pan, running temp is 150.
Dropped the pan and found the rear servo spring has pushed past the retaining clip, also found this metal disc in the pan.
Whats your opinion on what happened?