Coolant system issues

My MG F recently overheated. The mechanic who towed it for me said it was likely a blocked coolant pipe or radiator. I put a new radiator on it and changed the coolant pipes to stainless steel ones. The old pipes looked corroded and the coolant had bits of rust and metal in it. I put new coolant in it and it seems to be fine. Drive it for a while. Then it over heated again. Look like it seemed to have drained all my coolant then filled itself back up while I was stopped then drained it all. Also it seemed like it had an excess amount of pressure built up. Anyone got any ideas?

Did you fully bleed the system after refilling it? Might have an airlock if not.

Take off the bypass hose at the front if the engine (it's next to the temperature sensors) and check for any build up of crud in the metal bit. Mine was solid

It gives odd effects that are difficult to diagnose as the termination never gets to see hot water until the system is about to overheat

G'day and welcome, as a new member you may want to have a look at this checklist: https://www.the-t-bar.com/forum/54-how-tos/99326-a-check-list-for-new-owners

I fully bled the system but still had the issue. Seems to be flowing fine but building up too much pressure 

Is it building up too much pressure or is the pressure relief cooling system cap failing? they have a bad habit of doing that

The caps have a limited lifespan and the more they let by the worse they get. If you havent replaced it already it should be first on your list of suspects. There are some rubbish ones about so best buy from a proper supplier such as Rimmers or one of the specialists for these engines

I would avoid mg mania, I got one from them and it had minimal movement in the valve and after my test drive the coolant was about 3 inches above the max, I thought I had done another head gasket.

The ones from dmgrs have been fine, they didn't have new old stock last time I ordered but the febi bilstein one seems ok

The cap seems fine. And I have had it tested by an AA mechanic but might change it anyways. It seems to pressurize fine but when it over pressures that when I start having issues. I will have a look at the bypass hose as soon as I find it.

When you said you fully bled the system please confirm where from ?

A lot of people say this but have only bled from the radiator bleed screw, without going to the heater matrix bleed screw or the 3rd point which is on a metal pipe that sits above the starter motor & gearbox.

If you have an IR thermometer, check the temperature on the radiator hoses at the front and any pipes and hoses you can aim it at on the back. From a standing start how long does it take for the radiator hoses to get hot ? How long does it take for the radiator fan to switch on ? Does the engine bay fan come on at any point.

The system was fully bled. I have just checked the bypass hose and it looks fine. Anyone got any other ideas?

People are trying to help you but you also need to help them, please state which bleed points were opened, simply saying "properly bled" is not helpful. 
"The caps have a limited lifespan and the more they let by the worse they get. If you havent replaced it already it should be first on your list of suspects. There are some rubbish ones about so best buy from a proper supplier such as Rimmers or one of the specialists for these engines"
That's part of the reason that I changed to a VW tank, even then you need to be sure that you get a decent cap.