had the same problem a couple of months ago , when fitted my new braided lines,
it started to be intermittent at first then stopped working altogether ,
I didn't think it was the switch ,checked everything , wiring ,re-bled the brakes
in the end I got a new pressure switch and it cured the problem ,
maybe the extra pressure in the system nackers the old switches ,
Get a new switch cos it will do your head in iff you dont