Mountain bike tyres unreliably lose and keep pressure? I have a old mountain bike (10 years old) and for most of it's life it's sat in the shed (being ridden lightly maybe once or twice a year), only recently have I tried riding it regularly for the first time since ownership.
I have a really strange problem with the bike tyre pressures, sometimes whilst it has been in the shed they have gone completely flat and just sat there like that for over a year.  When I finally get round to riding it, I pump them up and they are good to go, however, sometimes after pumping them up they go completely flat the next day.  Other times, when I pump them up they hold pressure for months on end (if not years) without going flat.
What I found really strange is yesterday I thought my front tyre pressure was slightly low so thought I'd pump both tyres up nice and high as I'm mostly using the bike on the road now.  Pumped them both up, put the bike back in the shed.  Today, I go for a bike ride and the back tyre is perfect, solid, the front one is completely out of air.  So pumped it up again and went for a ride, half way on my journey it went completely flat again and so had to ride back on the flat.
Now I've taken apart the front tyre and pumped up just the inner tube, it's holding pressure fine for now, will do the water bucket test tomorrow, but there are currently no signs of leaks.
Does anyone have any idea why my front tyre can hold pressure for months on end and then after being pumped up it immediately goes flat very quickly sometimes (the back has done the same in the past).
Since I'm riding the bike more often, I really need to sort this as I am travelling further and further and it really sucks to come back on a flat.
Sounds like the butyl rubber inner tubes have perished a little, and you have either a very slow leak, or a valve that lets the air out slowly.
Just buy three new tubes from an online supplier - they'll cost about $5 each.  Two to fit, and one to carry as the spare.  You should also have two/three plastic tyre levers, a working pump, and whatever tools are needed to get a wheel off the bike.
The old tubes can be used to hold plants to stakes in the garden, or as a chainstay protector, or sliced up into rubber bands.
Also, I'd suggest having a close look at the tyre's surface, and decide if they're getting a bit old.  Rubber is attacked by ozone, UV, and heat, so if your bike was stored in a shed/garage then they can slowly deteriorate.  If you're doing strictly road riding then its a good chance to change to more efficient tyres too, compared to MTB knobblies.
Finally, check your brake pads.  Rim brakes have rubber blocks which can harden with age, so your brake performance can suffer.  Disk brake pads are not rubber so don't deteriorate in the same way.
These are all consumable items, so don't get put off by some costs.