I never thought I'd bae asking question on schooling the canter

Anyway... I discovered today that last weeks cutting in on the circle wasn't a one off. However bearing in mind that Pinto has a spavin on his right hock, I think that the left canter depart might be physically difficult for him, so I'm looking for advice on how I can make things easier for him.
He is keen enough to canter, and will lift well into the canter on the right rein, but on the left rein he will turn away from the track sharply and end up on a 7-10m circle instead of a 20m circle. Then he becomes very difficult to push back out onto the track without leading his nose in that direction.
Pinto is very sensitive to weight aids, in that he will turn into the weight, so I don't know if some of it is me, weighting too much the inside seatbone, but since I'm trying so hard to keep him on the track, I don't know if I'm now "pushing" him off the track, with too much weight to the outside, even though I don't think I'm leaning.
Any advice would be gratefully received

thanks
