Chombi1 

On a tubular tire, where it goes into the base tape?
Would an injectable sealant work to seal the leak?

CroMo Mike 

In my experience, no. Also, often what appears to be a leak at the valve stem is actually coming from some other place. Air leaking inside the casing comes out at the easiest place - the stem hole. Once I tried sealant and it came out around the stem, and began to fill the rim. Not pretty.

Moe Zhoost

Yep, air from a tubular tube leak will always migrate to the valve stem, confounding locating the puncture. Get your magnifying glass out and search the tread for possible punctures if you want to have a chance for real repair. That said, I have used a sealant somewhat successfully on a latex tube with a very slow leak of undetermined location. Effetto Mariposa Caffelatex was the product. Good luck.

repechage

Old old tubulars had a valve stem bolted to the tube. ( completely threaded valve stem required)
more modern tires will have a molded base bonded or vulcanized to the tube.
a leak there is probably a tear
those though usually deflate fast.
one check with the tire off the rim, inflate till it turns inverted almost and manipulate the valve observing for faster deflation.

so, what was also mentioned as a leak not revealing at the puncture is true.

I have a few tires where the leak just does not show anywhere. But overnight flat.
a good candidate for sealant.
thinking of the Newer Silca product.

El Chaba

If the leak is actually from the valve stem area, count me as another vote for the tubular mortuary. If the air is coming from elsewhere, you might be able to patch it in the normal way, should yo want to go to that trouble. My bet would be on a valve stem leak as that is not a rare thing, especially with threaded valve stems as they take a bit of a beating from the pump head being mounted and removed during inflation…

Classtime 

Pinch the tubular in various places to restrict the air as you pump it up. This can help locate the puncture. You can fold the tire or use pliers, vise grips, etc. I doubt it is the valve.

Chombi1 

I submerged the tire while pumping it up and the bubbles are coming from the seam where the base of the valve stem goes ibto the base tape. I examined the joint closely and could not see any signs of a pin hole so the leak must be very miniscule and very hard to find and fix..... and as also mentioned could actually be happening deeper in the tire structure, away from the valve stem area .
I guess the tire is just toast at this point. The tire has a more modern construction that does not have stitching exposed like older tubs do. Everything seems to just have been permanently bonded together with glue.....