The Myth is - an oil leak between two bolted parts of a hydraulic component can be fixed by tightening the bolts.
This one, like a lot of other myths, stems from human intuition. It is natural for us to think that if there's a leak then something's loose and should be tightened up!
When you have two "oil-filled" parts bolted together (like, for example, the pump case and the end-plate) - there's an elastomeric seal between them (read - an O-ring or a gasket). And if the seal has "sealed-out" - tightening the bolts that secure the two pieces together, and most likely are already tightened up to correct maximum torque, will only achieve one of the two things - it will either stretch the fastener thread or it will damage the thread in the hole. Not good.
A few examples off the top of my head: gear-pumps, steering valves, gerotor motors. Oh yes - let us not forget the "red classic" - the Kawasaki K3V112 - which has about a million O-rings between the bodies, and since this pump most of the times is installed on excavators, which already run their hydraulics at high temperature, sooner or later it will spring an oil leak due to elastomer hardening - and the first thing a mechanic with "enhanced intuition " will do will be - look at the leak between the case and the end cap, get a quality Allen key and a big long pipe, and give the four big bolts securing the end-plate a nice "boost". Classic!
Just opened this PVE21 yesterday - yep, that's right - see the aluminum thread that came out together with the respective bolt? That's right - someone has been fixing leaks here... (Don't you love the imperial threads?)
So, if there' s a leak between two bodies - replace the O-ring (or the gasket). Don't try to fix it by tightening the bolts that are already tight.