Bicycle Mechanics - Continuous slow leak from both tires? Help

I have to pump up my tires at least once per week. To 90lbs from typically 40 or 50. Is this normal for 26" tires??

My tires need refilling about every 4 days. Goes from 90lbs to about 40lbs commuting. IS this normal or do I have a slow leak?

Thats alot. Mine go from 110 to 100 in 5 days.
Check your V stems to make sure they are tight.

That is a lot of loss.

On my road tires, I expect 1 - 2 lbs per day loss or less.

Mtn bike tires will stay for weeks at proper pressure.

I notice it in my car and bikes, when the temp changes alot back and forth it seems to have an effect on my air pressure.

Not typical if your using standard weight tubes, but would be if your using thin ultralight tubes. Check to make sure the schrader valve inside the stem is tight. Other than that you may have to remove the tubes and check for leaks, which would be very small so difficult to find, so you would have to air up the tube so its fairly large in diameter than dunk in water to find air bubbles, but start with the stem in the water first if tightening the valve dosen't solve it-I have a feeling it's there.

On my road bike I have to pump the tyres every time I want to ride as the pressure gradually falls . It´s the price of using thin , lightweight tubes but I am rather suprised that your pumping MTB tyres up to 90 psi as that´s not that far from the 105 / 110 psi f/r I use on my 23mm racing slicks ( I weigh 67 kg ) and many people I know run higher than that but off road you tend to run much lower - nearer 50 psi . What tyres are you using ?

My tires need refilling about every 4 days. Goes from 90lbs to about 40lbs commuting. IS this normal or do I have a slow leak?

Butyl or Latex tubes, Whats the recommended tyre pressure printed on the side of the tyre? As if it is overinflated, with a schraeder valve, then some loss of pressure is inevitable. Presta's do keep pressure better.

I ride 26"tyre with slicks inflated to 90 lbs, Butyl Tubes with presta valves will keep this pressure and not lose much over a week. Latex on the Mountain bike will go from 60 lbs to around 5 lbs in a week. Could also be that you have a leak, or a thorn in the tyre and tube that is still sealing, but just, old patch leaking, or even the valve not sealing correctly.

My tires need refilling about every 4 days. Goes from 90lbs to about 40lbs commuting. IS this normal or do I have a slow leak?

It's a slow leak. If you keep riding on it, the leak will probably get faster. Check for anything very small lodged in the tyre that might have punctured the tube.

Dumb question, but I gotta ask... don't hate me.

What kind of pump are you using and how are you checking the pressure? Is it possible that you're letting a bunch of air out when you remove the pump nozzle?

ITs a whatever Joeblow pump with a lever let off. I will check the tightness of the valves.

Pull the tubes out of the tires and submerse them in water. Only submerse a small area at a time. Watch for little bubbles. There will be your leak. It certainly isn't normal to lose air pressure.

If you want to check the valve cores just use the spit test. Spit on your finger and wipe it over the valve stem/core. Does it bubble up? Yes/leaky valve core. No? Look elsewhere.

After you solve your leak prob get yourself a high quality floor pump with a gauge you can trust. Pump your tires to the right pressure before every ride. When you come home hose off your rig and brush the drivetrain and brakes wish a dish brush. Examine the tires for any little sharp glass bits, staples etc. stuck in the tread and let enough pressure out so they are soft but not riding on the rims, hang up your bike. This routine takes 10 extra minutes and prolongs the life of not only your tires/tubes but also your drivetrain.