I thought I remembered you posting the same query a while back.
http://www.cleanitup.co.uk/smf/index.php?topic=148788.0I presume then that the spare pump didn't solve your problem.
A shurflo pump has a small non return valve in the pump head activated with a fine hair spring. Its purpose is to prevent pressurised water from being pumped back to your tank. It is responsible for the pumps inlet and outlet path. (Water can only travel through the pump one way.)
You can suck water through the pump easily but can't blow it back as the non return valve plate stops this.
The spring may have a slightly higher tension in one pump than it does with another.
I still have the same pumps on the van since the first post - 1 drips occassionally and the other is almost a trickle, especially when the tank is full. They have done this since new. They haven't got worse with age.
The solution is to use a female Hozelok stop connector at the van port.
SIL has one of these and it has been drip free for the last 2 years he has had his wfp system.
http://www.gardinerpolesystems.co.uk/acatalog/hozelock-type-plastic-endstop.htmlI certainly wouldn't worry that you have a faulty pump - you haven't.