How do I find a coolant leak? Visual has found nothing. The reservoir needs filling every 50 miles...but I find no leaks. I have a modified cap for the tank, so I can pump air into the system to get it under pressure. How much psi do I pump in?

This is a lot of juice to lose. If it's not leaking to the ground, it's going into the crankcase or out the tailpipe. The system should be tested up to 15 psi.
Try pumping to 5 psi and then start the car and you'll likely see the gauge rise further.
I usually test the system including the cap by Teeing into the reservoir hose.
Ed
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'51 R25, '65 R69S, '67 1600GT, '01 V70 24T

Before you pump it up to 5-6 psi, check the heater core and the right side of the radiator (passenger side). It may not leak until the engine gets to operating temperature.

What are you pouring in every 50 miles? Just distilled water? Water boils at 212F, but the coolant mix is closer to 250-260F if I remember right.

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Keeping it running is better than buying new