That joint leaking and them telling you its "fektary faalt" of old cars is a pile of fresh bull potty (as capsat says) - Most probably either they leave the joint loose or you are missing some line clips for the liquid line which make the line stay attached to the body and not move.
Also please note that climate temperature is what dictates system pressure, if its blazing hot - so much so that even insects are using umbrellas to cover themselves then your system will also have a hard time working good.
Firstly dont cut corners in an A/C system, it should be perfectly working with matched parts, secondly you should look in the interior for any air leaks, primarily the hot air leaks are the missing plastic sheets in the doors or torn door rubbers, Your A/C cannot cool down a car that has such leaks. Also make sure your roof is not blazing hot from inside and that you have the original correct insulation under the carpet and behind the door panels too.