Cooling problems

hi guys, the cooling system on my disco td5 creates to much pressure and the top hose goes hard and I am loosing water through the overflow pie and the cap on the header tank. I have replaced the thermostat, header tank cap, water pump. I also had the cylinder head off, got it pressure tested and skimmed but still it develops to much pressure in the water system. I have also just done a Co2 test and result was negative. Under normal driving conditions, the radiator is staying cold all down one side opposite the top hose and In fairness the water pump I put on was a £20 cheapie from eBay. Is it worth spending some money on a new rad and good quality water pump or can anyone else think of other reasons why I am having this problem.

Ps, I am bleeding the water system by holding header tank as high as possible and filling with the bung out of the top hose while the engine is running

If you are getting the cooling system under a high pressure then it must be pressurised by something.
What are the options?
Head gasket failure,
Dowels (plastic ones need changing to steel),
Head cracked,
Bore or liner cracked,
Overheating - thermostat,
any more????
Is it only going it a full temp?

CO2 test didn’t show anything so I hope that cancels out head gasket, cracked head or bore, plus the head was pressure tested and new head gasket fitted. Also changed to steel dowels when I done the head gasket. My train of thought, if a lot of the rad is blocked and water isn’t circulating properly it might be building up pressure. The heater doesn’t blow what you would call hot air, just warm. Wouldn’t be hot enough to keep you cozy on a cold day

Had the same problem on my V8 turned out to be the water pump even though it was only a year or so old. Once stripped out, the water impeller was just spinning on the shaft so was just managing to provide enough flow at tickover to keep the engine coolish but as soon as the vehicle was driven the temperature would shoot up quickly. Also had no heater. Once the new pump was fitted the car was transformed. Something needs to be done about these rubbish pattern parts flooding the market, I purposely paid extra to get a metal impeller pump as I heard the plastic impeller pumps were prone to breakage or the impellers fell off, seems the metal one's are not much better.

Yeah, I’m thinking it’s a ****y water pump. Done another CO2 test this afternoon and it’s still showing clear. Something I did notice today was, when driving and a good few revs on the engine the heater was blowing warm but not hot air but as soon as you came off the gas going down hill or stopped in traffic the heater went cold. Think it’s time to get a good quality pump

Defo check water pump mine failed but took head gasket with it same symptoms as yours motorway speeds cool as a cucumber but as soon as slowed down she started getting hot
Impeller was loose on shaft
Also check drive on power steering pump isn't loose and the thermostat is fitted correct dog leg on bottom towards engine

Hi I have the same problem with my td5 disco changed the water pump and flushed the system then noticed that the viscous fan weak with the engine running you can stop the fan by hand (TRIED CARDBOARD FIRST ) so now have the problem do I replace the fan unit or fit a electric kit ??. Living down here in the west country we have a few hills lol so need something reliable any suggestions also changed the thermostat so it must be the fan unit .