I noticed the same thing on mine about a month ago. I checked the fluid and it was low so I drove the truck to NAPA to get some more fluid. When I got home, I popped the hood and found that there was fluid leaking from the front top edge of the master. I pulled the cap and noticed that the whole top edge of the master was rusted pretty bad. I'm guessing that the rust isn't allowing it to seal properly. So, I went back to NAPA and ordered another master.
I haven't put it on yet though, so the truck is just sitting in the garage......I don't want to drive it on the salty roads right now anyhow
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'72 C-10
350 w/Lunati Voodoo cam (207*/213*, .437/.454), Performer intake, 625 Road Demon Jr. and Hooker headers.
Support your community, support yourself. Buy local, buy American (or Canadian).