Adding air to the shock will improve the ride with a load and reduce the likelihood of the shock bottoming out when hitting a bump with the bike loaded. There was a discussion here recently about how to calculate the correct shock pressure. You might want to do an archive search on it. The reason you shouldn't use a tire guage on the shocks is because of the very small volume of air in the shock. When you use a tire guage you allow some of it to escape and you lose pressure. It's best to use a small hand pump with an integral guage to pump up the shocks. DON'T try to use the tire air pressure at the filling station... it's way too high pressure and because of the shock's small volume you can easily exceed the 40 lbs max that the shock is rated. There was a discussion on here some time back about what hand pump some folks were using. That I remember they were not expensive and worked well. Maybe somebody will jump in here...
If you weigh 250 and your wife is 130..that equals 380 lbs..so you should use 10% of that in air..or 38 lbs of air in each shock. The maxium psi for the shocks is 70 psi not 40..I use a small electric compresser to fill mine and fill it a few lbs over...sence my pressure gauge lets off some air when it's pulled away...The important thing here is to use the same amount in each shock..So if you see your gauge after a few tries lets out 6 psi when you take it away..fill each shock with 44 lbs and use your gauge to "bleed" off the extra air.When riding with your passenger..set the shocks to #4..when solo ..turn it to #1 or #2.. Knifemaker
Several questions have been asked about how much air to put in the shocks...After examining what others have found works for them and comparing it to their weight...I have come up with a very simple guide: Set the air in each shock to 10% of your weight. so if you jump on the bathroom scale with all your riding appearal on and weigh 175 pounds...put 17-18 psi in each shock.( the actual factor worked out to be 0.9652...but 10% is close enough)
If taking a trip with a passenger and luggage..use the same formula..but remember not to exceed the 70psi that is the shocks safe limit.(that would be 700 pounds ...the max tire load is 490-something) The problem with the front forks is just the opposite. The springs are not stiff enough and the dampening is weak. Several here have installed the Progressive springs and used 20w fork oil with much success. Those with older bikes that have the ability to add air...can still benefit from some stiffer springs...
For solo riding or riders under 185 lbs , the shocks don't realy need any air. The basic table I have used and others here seem to follow is use 10% of the total weight for your psi in each shock...meaning if you weight 225 pounds..put 22 psi in each shock. If your ride seems stiff,,adding more air will not help much. Since your shocks are not holding air..you might want to check where it's leaking from...because you might also have lost some oil. When you sit on your bike with it upright and straight , the rear end should settle down about one inch. If it doesn't seem to move much at all...your springs are too stiff. This the the problem I had with my stock shocks..so I bought a pair of Progressive 412's off a member here. Helps alot. If the shocks air leak is in the fittings, you might be able to replace them. If it is leaking from the seal..I'd get a diffrent set. (I have a set of stock shocks if anyone is intrested that hold air..) The shocks should never be filled with a high pressure hose, as this can damage them. Only use a hand pump and do not exceed 45psi.(the internal volume is so small that it is very easy to over fill them with a gas station hose) As far as I know...the stock shocks are not rebuildable..but you might be able to replace the schrader valves if that's where the leak is. KM
Rear shocks air pressure:
Useable range: 0 - 43 psi Maximum: 71 psi (may cause damage) To get accurate readings, requires a gauge that does not loose air when applied (most pen-style pocket gauges loose air). I have a Victor gauge, about $5 at AutoZone.