91 Nissan D21 Pickup Leaking from behind the Master cylinder

I own a 91 D21 Nissan Hardbody Pickup with about 173 thousand miles on it. The whole front disc brake, calipers and other parts were replaced about two months ago and then the rear drum pads and wheel cylinders were replaced about one month later (over the thanksgiving holiday.). A local brake shop did the front brake and I did the back myself.
Shortly after having the whole front brake assembly redone I noticed small puddles of what I discovered was brake fluid appearing right behind my front driver's side wheel well. I did a little investigating and found the source to be between the back (facing the firewall) of the master cylinder and the housing for the brake boots (I don't know what it is technically called). Anyways I was about to call the place where I had my front brakes done fixed then the spots stopped showing up and the leak stopped.
The leaking never started back up when I fixed the rear wheel cylinders and brake pads and I went back to school (I am a college student) with no apparent problems. I was home for about a week and it leaked one night. Then I drove it about 2.5 hours and let it sit for two nights. It didn't leak on the first night, but did after the second. The brakes are not spongy at all and I have had no problems stopping. I am just confused as to what would be causing the problem. It does not leak while driving and does not leak right after turning off the ignition. It has to sit for at least 4 hours before I see anything on the ground and it only leaks once and then won't again until after it is driven again. Any ideas and suggestions would be greatly appreciated. The truck is 2 wheel drive with the six cylinder (3.0L) VG30E engine in it.