I would contend that the 800 to 1200 psi of the ps pump may easily push past the dust seal on the master cylinder, and the u-cup seals on the piston freely allow fluid past them. That's how they self adjust for wear. They only push one way, and towards the front is the way they push.
As stated in my reply to your PM, the issue is whether or not yours has a "weep hole" between the two units. I have heard some do, but I don't recall one on mine. If it does, you should be able to see it with a mirror on the bottom where the two join. If it does NOT have a weep hole, the ps pressure leaking past the front booster seal would have no where else to go but into the brakes.
Again, I have heard of this happening before.
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01 24v 100hp tips, 4"ss BDbrake &steering brace PSCpump 1.5"dom steering tapped box 1.5"ram manual CAD switch dual carters 3.55's LSD, big shaft nv5600/241HD 6 35's inners pulled in (no spacer) Ugly black and thrashed!