I have broken 20-30 master links with mine, as well as the slide bar twice. Truth is, it is a crummy design. Worse truth is that they were still offering the same design in 2004!
Just recently I decided to overhaul my K650. I have a small thread going in fabrication, called "K650 Backhoe Restore" that might shed some light.
Basically, the arm is going too fast. If you have a multi-speed PTO, make sure you use low - the 540 rpm one. You can feather the control to get the arm swinging, the problem is when you try to stop it, it stops all at once and breaks something.
Some time ago I put a needle valve on one of the swing cylinder hydraulic lines, but that didn't do enough. The side with no restriction would still let the oil rush out, and break the link when you stopped. Now I have two flow control valves - one on each side of the cylinder. They cost about $55 each. They have built-in check valves, so they just restrict the out-going flow on each end. I have them adjusted so it takes over half a minute to swing stop to stop in either direction. Better slow than broke.
I'll make it home later on in the week, and proceed with the restoration. First I'll give the hoe a good workout digging rocks to see if I have solved the problem. If not I'll be adding a couple of cylinders to make it work like a "real" backhoe.
Despite its shortcomings, I have done a lot with the backhoe in the past. You get pretty good at starting the swing with a full bucket, and smashing it into the ground before you stop it.
I hope this helps.
