Aftermarket aircon install

[Meerkatmag]

Hi all, new to this forum. I have an old transit that never had aircon as an option. I modified a power steering mount to accept a compressor and fitted a complete air con system. Gassed at a garage with genuine r134a gas. All works brilliantly... except its wearing through belts extremely quickly. After a couple of hours of driving the belt is already noticeably looser and has sunk into the v groove. The tensioner is not slipping, rather the belt is wearing down and slipping deeper into the groove. Over the summer i got through a whole belt, tightening it each time it felt loose. New belt on now and it's already starting again.

I've checked the belt alignment and as far as I can see (and measure) it's straight and true. Also there's no movement or vibration I can see in the compressor. Looking at it rotate it looks absolutely perfect. Belt loosens even without clutch engaged, just free spinning.

Any ideas what could be going on?

Sanden sd5h14 Compressor
Transit 2.5di 1986 engine.
13x1300 v belt running in the ps groove of the crank pulley.

[tbirdtbird]

Correct size belt for the pulleys involved?
Absolutely sure belt alignment is 100%