hey, I have a similar situation.
My S5 radiator is now leaking around the top passenger side of it.
Im thinking about my options.
1. Swap with S4 radiator. (I have one.)
2. Buy a new S5 radiator from a parts store like AdvancedAuto. (How much would this be?)
3. Upgrade to a koyo.
(I know this will be expensive.)
For now Im thinking of going with #1.
Ive looked over the differences, it would be a frankenstein system but I think it might work.
But I wouldn't have a cap at all.
My S4 radiator doesn't come with a cap. It dose have a sensor and overflow tube coming out from the top thats unscrew-able. That would be my point of fill up. I would also fill it up by taking off the upper hose at the thermostat, just to be sure. Ill still have the S5 overflow tank.
Think that might work until I get another S5 radiator?
Will not having a cap be such a problem? lol.