I have a 93 Chevy Silverado. Recently had an issue with the "check gages" light coming on. So I got a new sensor and new O ring on the filter. "Check gages" light went away but then now when I'm driving about 10-15 min. the oil pressure starts falling slowly, and when I brake to stop goes all the way down to zero but as soon as I hit the gas the pressure goes up to the middle. Have had my mechanic look at it 3 times. He thinks its my cluster??

To verify you should do a oil pressure check with a mechanical gauge to insure it is not the engine.

How do you do that? I'm thinking of taking it to a different mechanic and have them do the pressure check. I don think the other mechanic did that he did check my alternator and battery and everything was fine. He seems to think its my cluster and told me to just watch my fluids and that the trucks old. If the pressure id found to be fine then what do you think it might be??

By one of those cheap mechanical oil pressure gauges from the parts store and install it to verify the low oil pressure. Or take it to a shop and they might do that themselves. If you have low oil pressure you could try changing the oil pump and strainer or change to a thicker oil. If neither of those work you will be looking at a rebuild.