BLUF: yes, the design of the pin that pulls on the piston rod could be improved, but it's easy to work around the problem, and I'll show you how. Now the review.
If you are like me, before buying a product you look at what percentage of reviewers have given it a one star rating. Then you look for common threads. The common thread on this product is the famous pin, that tends to fall out of its hole causing much frustration. A lot of users also get frustrated because they find it difficult to replace the pin, even if they find it on the floor after it has fallen off.
Other than that, the pump works just fine.
So, here's the solution. You'll need a small vise grip (locking pliers), a bit of thin rubber or cardboard, and some contact cement (see last picture). A woodworking clamp helps, but it's not absolutely necessary. The steps: 1. Squeeze the pump to the bottom of the piston stroke and lock the pump in position (see first picture. You can use your hand, a bungee cord, a woodworking clamp, or what have you). 2. Wrap some thin rubber around the piston rod to avoid marring it and clamp it in place using the vice grip. This will prevent the spring from pulling it back. (See second picture). 4. Remove the woodworking clamp or whatever was keeping the movable lever arm in place. At this point it is easy to remove the movable lever arm which will expose the roll pin (see third picture). In my case it looked like the roll pin was pretty solidly in the hole (enough friction), but I can see how manufacturing tolerances and temperature excursions may reduce the friction between the pin and the inside of the hole. So I took a tiny drop of contact cement and put it on each side of the rod where it makes contact with the pin. BINGO. The friction between rod and hole is enough that the pin shouldn't fall off unless it is forced out of its hole. If at a later time you need to rebuild the pump, it is relatively easy to overcome the resistance of contact cement, so you haven't done anything that can't be undone.
Now you know this simple trick and you can deal with the minor design flaw. Four stars because this simple workaround should not be left up to the user. Otherwise, a handy dual purpose pump.
