Tires slowly lose air.

1986 Radian yx600. I believe they are tubeless. I am normally wrong about things tho.

 

Rims are tubeless.

 

It's very likely the tyre shop didn't clean the bead seats properly - very few of them do a proper job of it.
The result is ususally a leak sometime further on, just as you've found.

Then there's the possibility the tyre shop didn't check / replace the valve stem or the seals on it - again, hardly every happens. Who knows how many bikes are running around on 30 and 40 year old valve stems, just waiting to leak. Ancient valve stems are easy to refurbish, but they don't bother.

There are a flood of sub-standard valve cores on the market, might be that's the fault.

You need the soapy brush.

Right now I'm being plagued with a slow leak on the front, and I'm my own tyre shop - lazy bastard in it.

 

As others already mentioned, its either the bead seats, inside the rims, or its the valves/cores, or some of both.

I had this issue on my rear tire, so at the last tire change I removed the tire myself and thoroughly refinished the bead seats - not fun but not too difficult. There was a nasty layer of hardened goo, probably years of bike shops throwing glue over the rough bead seats to make the tire seal. I replaced the valve then I took it to the shop to mount a new tire (that's a small expense I don't mind).

If it were me, and the tires have plenty of life left, I would consider removing them and doing the above.

 

I am always way slow at mounting new tires on wheels but it is because I am pretty anal about cleaning the wheels before the new tires go on. I generally use scotch brite pads all the way around to get the old goo and corrosion off.

 

Aluminum Wheels can have porosity and thus slow air leaks? I had some Toyota wheels that had that problem. Had them blasted and powder coated (on the inner surface as well) solved that.
Cheers, 50gary