I have been having exactly the same problem, except that I have a new radiator, new heater matrix, new hoses, new thermostat, new water pump, no visible coolant leaks any where, but after about 7 miles the temp gauge reads just under the red. I have been topping up the coolant every day hoping it is just an air lock slowly working it's way out. There is no mix of oil in the water or water in the oil, so I wouldn't think it is hard gasket related?
Is your rad hot all over or only in parts ? Maybe a baffle problem ...
No, I haven't, going to check the thermostat tomorrow (if it ever stops raining) and see how I get on from there.Have you tried it with no thermostat in ?
I think it's fair to say that's goosed lolRight another update, so I put a new pressure cap on it yesterday, I have now pressurized the system again just a little over 15psi and, more LEAKS!
Here are some photo's of the radiator and a leak in the pipe going towards the heater matrix, me thinks its now deffo a new radiator and pipework.
View attachment 96321
View attachment 96322
View attachment 96323
View attachment 96324