bought a 51 reg doblo. Runs fine but puts a lot of pressure in header bottle and loss of water. Changed the bottletop but ran rough . Got a 2nd hand head changed it. STILL the same :bang:.How can i prove it the head the porroblem without a pointless test on it ? And buy a new head anyway and still have the same problem.For £530+vat and a waist o time. help.