Hi Alan,

Obviously any recommendation and information provided cannot be taken as an absolute and will depend on various onsite conditions and knowledge which are not available to me. As such, before you endeavour to be "hands on" please get a registered plumber to have a look and confirm, even carry out any plumbing changes.

To answer your questions:

1) No, if you add 7.6 and the relief valve is rated below this it will start leaking (relieving pressure). You need someone to calculate total possible pressure in the system VS total achievable pressure and both valves (incoming and relief) need to match. Where the fixtures are within that available head also plays a role.

2) No, As per #1 you risk the overflow running or even exceeding the total system pressure which worse case scenario will result in your hot water cylinder bursting.

3) relief valves are not installed at the road, you may have this wrong and may be referring to the limiting valve? In any case this reinforces that you should  get someone to look at your conditions and provide professional advice.

Another thing you may want to consider if you are looking for higher pressure is to simply replace the cylinder with a mains pressure tank. They are as cheap as 1900 installed these days. Any money you spend on valves etc could be going towards a new system.

I hope this helps.