Hi, John:
First, you need to establish where that water is coming from/venting. It is exactly the problem l had with my M Special when l first got it, back in 2015. I was losing large volumes of water, but couldn’t determine where the egress point was.
John Peskett id/d my rad as an early type and l was able then, to ascertain that a ball-valve at the top of the rad was missing, thus allowing water to vent, via the overflow pipe, straight to the outside as soon as pressure built up in the cooling system. That overflow pipe was well hidden : ), due to the nose cone of my car being such a close fit around the radiator. Only once l’d taken out the rad was l able to trace the outlet pipe; I had this pipe soldered up and a new, accessible overflow outlet grafted into the header which is a peculiar custom arrangement on my Special.
I then made up a simple expansion bottle and pipe system into which pressurised overflow can spill (and siphon back, as the car cools down after a run). This seems to work well, except perhaps on the hottest of days in standstill traffic.
So you need to ascertain where and whence your leakage point is. I assume that your head/block are not cracked. Does your car have a heater – in which case, might there be an airlock somewhere in the system?
Best of luck, Tim.