There are three components to pitot tube speedos in addition to the hoses (these do not include late model electronic units). From the back there are the pickups that develop water pressure, then inside the transom there are balance tubes (look like 3/4" x 1' brass tubes) they convert water pressure to air pressure, then the speedo's themselves.
It's pretty to easy diagnose the speedo's with a bicycle pump. I always start at the back pulling the tube off the pickup and pump air directly into the tube (may or may not require a fitting depending on your bike pump). If the speedo's register then you just need to clean out your pickup (the most common problem). Next, hook the pump directly to the speedo with a short hose and see if it's stuck, they almost always move indicating the problem is elsewhere.
The next most likely component is the balance tube, it might be stuck. A little more pressure from the pickup side might break the piston free. If not pull the hose off the speedo and pump there pushing the piston the opposite direction in the balance tube, then reconnect the speedo and try pumping from the back again. Still not moving? Locate and pull the balance tube and squirt a little wd-40 in the pickup side (not the speedo side). Keep track of thier direction, re-install them the same way. If you cannot get the balance tube piston moving replace it.
(Message edited by mikeski on June 26, 2006)