I Have the same problem, tyre goes down to about 10-13psi after three days of being pumped up, I changed the tyre on the car as it needed doing anyway and asked the guy to check the wheel for cracks, fine it seemed until 3 days later when it went down again? Went back to Slow-Fit to ask what is wrong couldn't find anything wrong no cracks or punctures. The guy said it could be the valve (which is what i thought), but was hard to tell as it doesn't have the same amount of pressure on the wheel as it's off the car. I also have angled Valve tyres on (I didn't buy them they came on the car). What they have recommended though is to buy Truck Valves as those are along the same line, i.e. angled. But not sure if anyone else has had this problem as angled valves seem rather uncommon.