Bicycle Mechanics - Tube leak down rate

I haven't ridden for about 10 days due to a combination of business travel and weather. Normally inflate my tires to about 120 psi due to my current 215 lb. weight. The rear tire was still very hard (>100 psi). The front tire was flat. Tubes are different brands since the rear is original and the front was recently replaced due to a flat. Would this much variation be expected in the leak down rate of different tube brands or should I expect a leak. Would the difference more like be due to the valuve design?

It's a leak. I'd check it out and see if you can patch the front. A slow leak like that means it's probably easily patchable and can be ridden on again.

We checked several wheels over a week(This was a couple of years ago) and found that road wheels(100psi+) lost between 4 and 8 pounds a day, tapering as the pressure dropped.
Bigger MTB wheels(60psi) lost 2-4 pounds a day.

You have a leak in your inner tube. You need to figure out what caused it or you may get frequent repeats.

Take out the inner tube, put some air into the tube and try to find the leak.

If the leak is on the inside circumference check your rim strip. If the rim strip doesn't completely cover ever tiny little arc of spoke hole, you'll get recurrent flats.

If the leak is on the outside circumference, check tire to be sure you don't still have a thorn or piece of wire or something stuck in the tire.

If the leak looks like two parallel slits in the side of the inner tube, whoever replaced the tube pinched a piece of it under the tire bead.

Thanks all. I did check the tire and rim strip when I replaced the tube originally (on the road), but didn't have my glasses with me. I felt around the tire and everything seemed fine, but could easily have missed a problem on the rim itself. Will definitely check carefully today. These tubes happen to be short stem anyway and don't seem to work particularly ell with my Blackburn pump.