brake fluid leak

I recently purchased a 1989 S-10 Blazer (4.3 V6). I am not a real motorhead so I will try to explain the situation as best I can. If I park on level ground I have no problem but when I park in my driveway (which is very, very slightly inclined up) the next morning I notice a 5" diameter spot of fresh brake fluid on the ground just inside the front driver-side tire. It is not leaking from the fill reservoir. It appears to be coming from the bottom of the larger thing next to the firewall. Why is it leaking and why only when I park slightly uphill? Could someone tell me what is happening here? What needs to be done to fix it? Thanks!

Are you talking about the big round black thing (brake booster) that the master cylinder (the metal thing under the reservoir) bolts on to? If it's vacuum operated, which I believe it is, that is not the source of the leak. Mine used to leak in that spot (doesn't now, the damn thing fixed itself), and it was the seals. Are you losing brake fluid when you find the spot on the ground?

Just reread your post and noticed the year. Yours is vacuum operated.

J-Ri: Thanks for the comments. Yes, I can reach under the big round black thing (brake booster...as you say) and find clean brake fluid on my finger. The master cylinder is dry. It is not leaking from there. I can see on the rubber shield (right under the brake booster) on top of the wheel well where it had been dripping a little bit during the night. I was in fact wondering if it was a seal of some sort, as you indicate. The fill reservoir does not appear to be losing much but I did add a tiny amount to compensate for what I found on the ground. If it is vacuum operated, as you say, what do I do now?

Just outta curiosity tonight I parked it with the back end uphill. 5 hours later no brake fluid spot on the ground. Why would it not leak when parked in a level spot? I drove it around this morning and then parked it on the street for several hours (in a level position) and no spot occured. If it were the seals I would think it would leak whenever, wherever. But then again I wish I knew more about car mechanics. Thanks alot for your input. I appreciate it.

Change the master cyl. It is leaking from the seal in the rear where it connects to the booster, You probably only notice the spot when facing uphill because the fluid is running back into the booster. When on the level or facing down, the fluid remains in the master cyl. If your going to try to change the master cyl. yourself, get someone to help you with a little experience, these things can be difficult to bleed and get the air out. good luck! Normally these things don't just stop leaking they get worse, but I have also had them stop leaking. (changed it anyways) Don't take a chance!

when you park with the front of the truck down hill the fluid goes to the front of the master cylinder, when you park the other way the fluid leaks out of the rear seal of the master cylinder.

new rebuild kit is under $30 or you can get a reman master cylinder for under $50

Thanks guys for your comments. I will replace the master cylinder. Perhaps a mechanic friend of mine will help me. Thanks again! Your input was helpful.