So I picked up the Peter Hunter half set off of ebay and just received it. All the pieces are there and it is playing-ish. All the joints needed rehemping (still working on getting the main stock air tight - the metal ferrule between the cup and mainstock seems to have been glued or something in the past, but it is loose right now) but it makes noise!

The biggest issue are the bag and bellows. The bag is a little leaky, but man the bellows are basically non-functional. If there is any back pressure (if you are playing a note or squeezing the bag at all) then all the air goes straight out the bellows along where the leather is stitched into the wood.

I'm new to the UP so I basically have stuck to the things I know from GHB (hemping joints) and haven't gone beyond that at this point. I have a practice set on order so I'm waiting for that to come in before I play - with the way the bellows and bag are I don't want to ingrain any bad habits.

In the meantime I thought it might be worth trying to rebuild the bellows. It seems to be only leaking at the seams so I should be able to pull the wood inlay to get to the stitching groove, remove the stitching and re-do it (I have a bit of experience leatherworking/saddle stitching). But I'm not sure what all needs to be done to make that connection air-tight.

I know I can pick up a Kelleher bag and/or bellows but...why not take a stab at getting this going? I may also recruit my wife to take a stab at making a bag from upholstery/marine vinyl.

It's probably good to create a seal between the leather and the wood with a thin layer of adhesive or caulk (making sure to use a type that won't damage the materials), in addition to the stitching.

Get some pipe bag seasoning before you do anything serious. You might find it gives you an airtight bellows. Also, replace the valves the set, that should make a big difference too.

D. M. Quinn provides some very detailed information with plenty of photos about the re-leathering of an old bellows in the facebook group "bagpipe makers exchange".
You have to scroll down until you reach May 29th, 2019, that's where it starts - it will probably take you quite a while to get there but it's worth it
Be sure you also read the comments, there's some valuable information hidden there.