Brakes

I have a 1990 Silhouette. I have put front and rear brakes new master cylinder, new rotors and drums, calipers and rear pistons on this van. Now my problem started one morning after a night of rain, my wife was leaving work and when she hit the brakes the rear right wheel locked up. She got home and told me about it and I went out to check it and it didn't act up on me. The problem only happend after it rained never while it was raining, so I ended up changing that piston and the problem went away for a couple of months until tonight. It has rained all day but when my wife went to work and was pulling out of the driveway she said it felt like the front and the back where braking unevenly. One more thing I did do was change the master cylinder again before I changed that r.r. piston. I'm at my wits end trying to figure out the problem and everybody that I have asked have been stumped, especially it only happens after a rain and the van hasn't been driven during the rain. I need someone with a different view on this. Thank you for any help on this matter.