The tension in the XT M772 9spd rear mech P-Tension Spring area has gone. It doesn’t spring back properly so the chain is hanging slack. I’ve taken the inner plate off but how do I get into the P-Tension spring area to try to sort it out?

There is a small grub screw in the knuckle on the mech, undo this and pull it out (it’s a pin) and the cage will come off. You can then replace the spring, or at least clean it out and plug it back into the holes (it can pop out).

Grub screw is rounded, can’t get it out. Suggestions?

If it’s a hex grub screw try undoing it with a similar sized Torx

Managed to get it out after soaking in wd40 and it was like a swamp in there, mud, dirt, water. Cleaned the lot out but noticed that the seal is missing so this will probably keep happening. Can I just stick in a seal from an old rear mech?