Fuel sending unit and how to tell if it needs to be replaced

I'd like to know how to tell if the fuel sending unit needs to be replaced.
Everything is fine when the gas is above 1/4 of a tank. When I hit the 1/4 of a tank mark, it seems like I only go about 30 miles or so when my fuel light comes on. When that comes on and I make hard left turns, the car acts like it's out of gas until I get straight again.
When I put three or four gallons in, it takes about ten miles before the gas gauge moves back up over the 1/4 tank mark.
I never had this problem before I replaced the gas tank, air vent solenoid valve and the vapor canister under the gas tank. I have always gotten about 390 miles to the tank in all three of my Proteges before this.
Any suggestions would be a great help! Thanks.

I really need some help- still stalling and leaking gas

I filled my tank up to 3/4 yesterday for the first time in a couple of weeks. Today I'm smelling gas inside the car, it hesitated on the way home and when I stopped to get the mail, it stalled, then stalled again at the bottom of the driveway and leaked about a half cup of gas.
I don't want to take it to the mechanic who's been working on it, cause he obviously doesn't have a clue, but I don't want to take it to Mazda cause it'll cost me more of a fortune, though they can probably figure the problem out.
I need some advice, please. This gas tank situation has cost me roughly $1200 so far and the problem is obviously not fixed.

Doesn't sound like the fuel pump to me.common signs of a bad one is it'll surge or lose power at high speeds or under Load.if youre losing fuel id look for a fuel leak first

Well, it did turn out to be a bad sending unit and the gasket around the fuel pump. The gasket was flattened out, which was causing the fuel leak when it was almost full to full and the sending unit caused the problems between 1/4 tank and the fuel light coming on.
The air vent solenoid and vapor canister were not needed, just extras I paid for.
So, if I had just brought it to someone who knew what they were doing in the first place, I would have saved about $500.
This is for all of those who have questions about bogging down, smelling fuel, etc....

The same thing can happen to an older Mazda 3 - sending unit gasket leaks and a gasoline smell is noticed. I think the access panel is behind the back seat.

Access is under the back seat.
An update to my problems. The other half of my leaking gas out of the check valve problem was, the gas tank I got off Ebay from California, had a valve door that either wasn't cut out or didn't open, for emission purposes I imagine. So, the lines that come out of the tank to fill the vapor canisters, were getting filled with gas when the tank was 3/4 to full. The canisters were getting filled with gas and had no where else to go, so a reverse vacuum occurred and gas was coming out the vapor lines and out the check valve. I went yesterday and got a used tank off a 2002 Protege in NC and am going to have them swapped out this weekend. And I hope that is the end of all these problems I've been having since I had the new tank installed in mid June. A new fuel pump and seal, and the correct gas tank. What a pain in the a**, all because the original mechanic didn't take the time to correctly assess and diagnose the real problems. Make sure you have someone who knows what to look for and where, otherwise you'll spend money for things you don't need, that won't fix the real problems.