I replaced the ac compressor , drier and expanction valve . I flushed the system and replaced the washers . But when I vacuumed the system it will come down to 28 but when I shut the pump off it will not hold the vacuum for very long . Which means I have a small leak what is the best way to find it ?

Bad news. You should put some refrigerant in there and get it sniffed. Do not leave it under a vacuum or air will enter and contaminate your dryer and possibly the oil.

You should also ensure that your gauge set is not leaking by isolating it and seeing if it holds vacuum by itself.

Thanks . I should of checked for leaks before i tore it apart . The gauge set is new but that does not mean its doesnt leak .