Car dies when braking

Ok here we go
with yet
another issue.
So the
serpentine belt
broke, my
friend puts a
new one on not
a week later it
snaps. Had to
replace
tensioner and
belt. The
tensioner was
locked up no
give at all.
Anyway
replaced belt
and tensioner
and that night
the car drove
great.
Yesterday
though it
started stalling
out on me
when I hit the
brakes. Doesn't
matter if I'm
just slowing
down or
coming to a
stop it just
dies. It starts
right back up
with no issue,
it is a little
sluggish on
take off but it
runs unless as
said I hit the
brakes. Its
been
suggested it
could be a
torque
converter lock
out, I don't
think that's it
just simply
because all I
have read
about that is
that signs of
transmission
slipping would
show first and
that's not
happened ever.
Also that it
could be the
brake booster
not a clue what
that is either.
Any ideas on
what it could
be and how to
check it out
even if its the
above
suggestions