Mmm, got a 5711 d4. Has developed brake issues recently. Had it a year or so. Minor niggles but been ok other than the brakes.
Right hand side seems to stick...noticeable on soft ground or up hill on the road.
On the whole I've found it to be a cracking tractor, but I'm comparing it to 25 year old stuff. As for reliability...ask again in 5 years
What became of the brakes? Ive heard of another doing this.