Check the oil level and top up if low, there is a cross mark on the side to indicate the level it should be, also ensure the reservoir is as near verticle as you can get it in its repositioned place in the boot. It could be that the pump is weak on one side if the oil level is correct, you could swap the pipes over on the pump with each other and see if that reverses the strength of the opening so it opens without help and locks on its own. If this improves its opening and locking and the problem is then seen on the closing cycle then it is probably the pump getting weak. Worth checking the bypass valve that the red pull cable operates on is fully closed as well, if it has stuck slightly open then pressure would not be 100%. Lube the pressure plunger on the pump body and push it in and out with a screwdriver so that it opens and closes ok when the red pull cable release is operated.