You had the rear master too full. And as for sitting and locking up????
If the rear master is too full when you go down the road and things warm up it expands the fluid and applies the breaks. As for it sitting did you refill it and over fill again and it is heating up someway to expand fluid??? You do not fill the rear master full. Fill to about 1/2 inch from top. Hard to see in there and if you get that brake fluid on anything it is the greatest plastic destroyer made and removes paint also!!! After you work on brakes on these things you always carry a 8mm wrench to relive pressure if it builds up for a few trips.