i get arm pump too. i found a cure for it though..... but i havent got it yet.
my friend chris is the brittish motox enduro champion.... along with other championships he has won. he went to belguim, in europe, for an operation to stop arm pump.
when they open your arm up, they take a layer of tissue off the top side of your muscle... right round your arm. this lets your arm mucles expand to there full and stops the arm pump.
i think it cost him