A 20 year old bike, have you changed the fork oil and cleaned out the forks? This can really improve the front fork action. I have changed oil and replaced the seals in hundreds of bikes and the oil almost always comes out black and contaminated. The fork air caps are there on new bikes to bleed off excess pressure (it will build up sometimes as you ride) on older bikes the caps are there to add air to augment the fork springs. In my opinion the best way to augment the front end is to space out the springs with a spacer. If you want to add some air to the forks use a low pressure gauge and add somewhere between 0 and 12 PSI. Another thing to look into is progressive brand fork springs. In the last couple of weeks I have put in 3 sets of these springs and each bike improved greatly in the front fork action.