PAC MAN: Edmund G. (Jerry) Brown Jr. has based his Democratic presidential campaign on attacking the influence of money on politics, but his own experience shows that big bucks don't always translate into support. . . . Brown set up a political action committee when he stepped down as California governor in 1983, and the next year it gave substantial sums to several Democrats. The biggest contribution--$10,000--went to the Senate campaign of Democrat Tom Harkin of Iowa. Harkin no doubt appreciated the gift, but it did not keep him from endorsing Clinton after ending his own presidential campaign last month.