The pump seems to be a pressure and flow compensated hydraulic piston pump. That means at a certain pressure the swash plate moves and changed its angle. By doing this the stroke of the pistons is reduced and so the flow decreases. But in this case the oil is not bypassed to the tank line but the flow is reduced down to zero. If you are sure that the flow (full flow) is going back to the tank when the pumps exceeds 50 bars, it will be either the pressure relief valve that is set to 50 bar or - as ash9144 suggests - the piston rings are worn.What brand your pump is? I have absolutely no clue Suber-water pumps. But companies like Parker, Vickers and Rexroth manufacture such kind of pumps. If you have a distributor of those nearby, ask them for help. 
 
I'm not sure this pump's mother would recognize this pump from this photo.  It might help if you posted some photographic shots from other angles as well.And some particulars regarding what it is connected to or what drives it.Information is not a bad thing.

2011-08-07

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