I am going crazy! I had an issue with my brake master cylinder losing pressure and the pedal sinking at stop lights. So, I bought one on ebay, installed and bled then the same problem started happening.  SO, then I bought a NEW MC and the same thing... Is there a trick with the bleeding procedure? I think I am losing pressure. I checked for vacuum leaks and could not find one... Has anyone run into this? PLEASE HELP!

Did you bench bleed the master cylinder before installation??  If not, you will continually push air from master cylinder to the wheel cylinder/calipers.  Once you get it bench bled, go to passenger side front wheel, then passenger side rear wheel, then driver side rear and finally driver front.  ALWAYS make sure your reservoir is topped off.  The reservoir on these things are TINY and require constant attention!! 

Good Luck

The instructions were very clear not to bench bleed this MC... it is a 1995 4 Door... there were multiple tags saying not to bench bleed with BOLD letters. Also, the rear is daisy chained... I thought you start at the furthest from the MC... is that not right?

Found the procedures for the specific model and specs... bled as instructed and still losing pressure... Ugh

The only other thing it could be in theory is a external leak! I would say go over your rig again and check for leaks.

or a warn rubber lines thats expanding when ur pressing the brakes

its pretty common with all cars some times they just dont hold up

also u are right mallard generally u bleed brakes furthest from the mc. unless factory but rarely says a different way