[rudidelange]

If all your oil seals are ok and the and crank pulley bolt is torqued correctly and still low pressure than it could be worn rod bearings, or the oil pressure relief valve. Oil pumps seldom wear out - unless there is some mechanical failure.
Clean the pressure relief valve (I used metal polish to remove the varnish), replace the oil with 10w 40. At idle with cold oil it should give 4.5 - 5 bars with ambient temp at 20 deg celcius. If still low increase the revolutions to 2500 - it should increase.
If still low check with an external oil pressure gauge. You can fit it at the same place of the sender unit - will have to place the car on ramps. It the external gauge stills shows 1 bar at idle then it is most probably the rod bearings.
If you have to remove the oil pump then you must remove the sump - big job!!!!
If the sump is removed then you can check the rod bearings - if you removed the rod bearings then you can just as well remove the top and replaced the head gasket - when the top and bottom is open - then you can just as well push the pistons out and check the rings - whats left would be the block - then you could just as well remove the engine and redo it ... ask me!!
Enjoy the car