"Dale, these most always work
"Dale, these most always work well if all components suggested have been examined to be in good working order.
The clear hose may give an indication, but I'd rather see you spend this time on going over your system once again.
The water neck fitting (hose connection) is a critical component... that and the proper installation of it's gasket and the S hose connection.
There is an "UP" side to the installation of this ribbed gasket, and it must be installed correctly.
Any corrosion powder within the S hose (from the previous water neck) must be cleaned from it.
Any loose suction hose clamps must be addressed.
Any bad copper tubing gaskets must be replaced.
The lower pivot tube O ring must be in excellent condition and in place.
I'd also suggest that pressurizing a suction system to detect leaks may not be the best approach. Pressure can sometimes "mask" a suction leak.... or even offer a "False Positive".
I'd still do it, but just be aware!
You could also connect a shop vac to the inboard side of the system and see if you can test via some method....... You'd have to get creative, I suppose.
Maybe this is where your "Clear Hose" idea, my shop vac idea, El P's masking tape idea and some colored soapy liquid may work!??? Starting from the drive, brush the liquid on... look for bubbles in the clear hose. I actually like that idea!
I still think you'll find the problem if you are vigilant in your pursuit by just going over the system once more..... and this would include the engine Circulating pump, Sea Water pump, T stat, Heat Exchanger, etc."