Thanks Namreg, will spend some time looking.
Well, an update and not all happy to be honest.
Part arrived thursday afternoon, as I was flying back to SA on friday evening, we quickly installed the valve and started up, machine sounded fine and the oil pressure which had fluctuated before is rock steady at 6 mega pascals.
Then, at around 120 bar the bubbles (froth) in the oil sight glass start again.
SO, we stripped out the piston and liner again thinking it was faulty and replaced with another, still frothing, checked the valves suction and discharge, seem fine, start up, machine sounds fine oil pressure steady, 120 bar oil froths.
Stripped oil pressure regulator, replaced o-rings all looks fine, bled the oil pump, rechecked - oil froths.
Now, we are left with limited options, -
1)Can the check valve on the final seperator cause this, we thought if the check valve is faulty,it mat allow air back into the oil return line?? - it may be a long shot, but can it be a cause.?
2)As only the final (fourth) stage is under oil pressure, can a fault on another, say the third stage, cause this. What could it be.?
Something we noticed now is that when we turn the machine off, the oil pressure only drops to about 2 mega pascals not to zero, like the other machine, one of the technicians here dosnt seem to think this is an issue.?
Something else we noticed was just before the oil starts to froth, there appears to be an audible "click" like a valve opening or closing, until we hear that click there is no frothing, so somewhere its connected.
Okay chaps, put your thinking caps on, wednesday I am flying back - gotta have some answers.!!