Thanks again.
Well it seems to be sorted now, Jake sold me his lever and kindly let me have his two slaves too.
I took everything apart, gave it a service almost cleaning everything up like the connections and it seems to be working spot on.
Rode 6 hours pretty hard yesterday without the pressure even dropping at all. I guess it could have been my slave seals that were broken or my piping wasn't secure enough.
Cheers for the replies