My 86 did EXACTLY the same thing after i got it back on the road,but mine sat for a while,it was the brake booster in my case,i replaced the master cyl and no changes,i had a scrap ranger sitting here and swapped that one out with it,solved the problem in my case.
My son has an 87 B2 all cut up and just used for the woods,his brake booster was leaking vaccumm real bad,he took my old one from my 86 and for the heck of it sprayed WD40 in the interior side of the booster and worked it loose,he's still using it in his with no problems but i wouldnt recommend that for a daily driver thinking the WD40 may hurt the seal or something in there.