I currently own a Serfas floor pump, but have a hard time getting it positioned correctly onto a presta valve so that the tire will accept air (sometimes I take the nozzle on and off 8-10 times till I get it right). Do I have a crappy pump or is there a trick to positioning the nozzle correctly?

And yes, I know with a presta value you have to loosen up the little duhickey on the vavle in order for the tire to accept air...

If I have a crappy floor pump, do you have any to recommend?

I have a pretty good Blackburn(older model forget #) Its all aluminum with a cast Al base, nice and solid with a good guage, but the air chuck gives me the same issues as yours, the weight of the hose and the air chuck twists the chuck on the presta valve and makes it a pain in the butt. I use a presta/schrader adapter and the wider diameter of the schrader seems to cure the problem.(the air chuck does both types)
They sell nice metal air chucks that I may get at some point.

Will it not seal on the stem?

I used to try to press my pump fitting down all the way because I thought that the fitting had to press in the valve stem. I have found that the fitting doesn't have to press the doohickey down to allow the tire to take air.

I always bleed a bit of air out of the tire before putting the fitting on. This creates enough of a pressure difference for me to be sure that air is transferring to the tire. When I build pressure in the pump I hear a tick sound as the valve opens and equalizes pressure from the pump hose with the tire.

I may be wrong, but it works for me that way.

No, I have no problem getting a seal. Actually what usually happens is that I get a seal (between the stem and pump nozzle) and end up just filling the hose with about 60psi before it starts to bleed out. None of that air goes into the tire unless I have that nozzle fitted on there just perfect. It's rather frustrating...

Most pumps have a lock on them that tighten when you lift them up. Like seen on https://www.parktool.com/products/det...=17&item=PFP-3
Not sure if the fitting can be changed on that pump.

yes, my pump has a similar locking nozzle. however that doesn't seem to be the issue. I mean it locks in place fine, it just doesn't always allow the air to enter the tire.

Try releasing a little air out of the tire before attaching the hose. If all else fails get a Joe Blow.

I always unscrew the valve and then press it to release some air before I attach the pump. Currently one of my tubes accepts air immediately and the other one acts like yours. I just keep pumping until it accepts air or I can't pump anymore. Occasionally it takes a second attempt. I have learned that if it has some pressure in the hose the valve will open up and let air in after a couple of seconds. Rarely do I have to make a second attempt since following this procedure.

If releasing air before you attach the hose does not loosen the valve enough to air up the tube without hassle then the tube needs replacing. As long as you have the chuck attached securely it is not the floor pumps fault. At least that has been my experience.

I always release most of the air in the tires before pumping. And my old Silca Super Pista pump has never failed me since I got it in '93!