Maybe I'm missing something on the basics of the problem first stated, but if you're pumping the fuel bulb (with gas tank valve selector in the proper position, gas in the tank, and the bulb getting and remaining hard), but gas is not filling the Racor bowl after changing the filter element, it's because you have air in the fuel delivery system, AND fuel in the carb bowls. If the carb bowls have a full load of fuel, their floats are in the "up" position, and neither air nor gas is going to flow through the bulb, no matter how long, nor how hard you pump. Therefore, no fuel is going to be drawn out of the gas tank, and no fuel is going to fill the Racor.
You can fix this either of two ways. First, as already suggested, separate the fuel hose at the motor, and use an electrical cable strap or something else to depress the ball valve in the connector. Then, with the ball depressed, the connector elevated, and the bulb in the vertical position, fuel and air can flow freely out the end of the hose as you pump the bulb, and you will draw fuel from the tank, displace the air in the system, and fill the Racor bowl. But, if you've already pumped the bulb, watch out, because the hose is under pressure, and you're likely to get a face full of gas when you depress the ball. And, since the fuel delivery system is now "open", watch out for fuel coming out the end of the hose, which is a rather crude way of telling when you've pumped the bulb enough.
Or, you can simply drain one of the carburator bowls, causing that carb's float to lower, and it's fill valve to open. Then, pump the bulb. The carb's valve will stay open, so long as you're just passing air, since there's nothing for the float to float in. Once the air is purged through the carb, and gas is flowing through the bulb and into the carb's bowl, the float will raise, closing the carb's inlet valve. At that point, you should have all fuel, and no air, in the Racor. If not, you pumped the bulb too fast. If that's the case, drain the carb bowl again, and go slower pumping. If you watch the Racor bowl while you're pumping, it's pretty easy to see what's happening. Since all you need is for the carb's valve to open slightly, you don't really have to drain it's bowl all the way. Try removing just a little fuel, and see if that's enough to open the carb's float valve. It's pretty east to tell if you've removed enough fuel, if you keep a hand on the fuel bulb. Once the carb's float valve opens, the bulb will go soft as the pressure in the fuel deivery system is reduced through the carb. At that point, you're set to pump the bulb.