braking problem '83 K10 w 14bff

[nikwho]

I have an '83 K10 with 3/4 axles swapped onto it. Its got a corporate 10 bolt in the front and a 14 bolt full floater in the rear w/ drum brakes. I was told that the axles were from a "comparable year" 3/4 truck. Truck was as is when I bought it. I am assuming that it has the stock K10 proportioning valve. My problem is that my rear brakes will lock up very easily with what feels like very little if no braking to the front. has, a couple of times, stalled the engine while braking in gear. Even with just the slightest push of the brake pedal. Makes for some interesting driving characteristics! Its a nightmare(ok, sometimes kind of fun) in the snow! Any thoughts? I was thinking that maybe its a proparitioning valve issue. Thanks for the help guys
Nik