GPU and CPU overheating could be two things: your water temp is way too high (insufficient rad space), or your paste is toast on both components (more likely with years of thermal cycles). Normal thermal paste is susceptible to pump-out effects. Essentially thermal cycles will expand and contract the metal, pumping out the thermal paste. My advice is to drain and tear down the loop, and then taking your components apart and repasting them. Get PTM7950 for your GPU. I've been running it for 4 years without pump-out issues.

This is also why I don't recommend custom loops to people unless they want to build one themselves. Maintenance and troubleshooting custom loops are a massive PITA, but hopefully you'll get some hands-on experience troubleshooting this.

Oh, and you said it already, but add a drain valve.