Suitability of flexible metallic conduit serving as ground
Re-pulling through installed flex is a nightmare. If you do, keep in mind that new-style dimers require a neutral. -EMT fittings? I'm guessing there's no red bushing either. 

Flexible conduit used as ground? - InterNACHI Inspection Forum:


  "In general FMC or flexible metal conduit is limited to 6' in length when used as an EGC (equipment grounding conductor). Since this is an old installation it's likely grandfathered in, however by modern standards it's could be considered ungrounded even though all of the components of the system are metallic. Is the stuff in the photo actually FMC or is it old AC (BX) cable?"
  
  250.118
  
  (5) Listed flexible metal conduit meeting all the following conditions:
  
  a. The conduit is terminated in listed fittings.
  
  b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less.
  
  c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). (does not include AC)
  
  d. Where used to connect equipment where flexibility is necessary after installation, an equipment grounding conductor shall be installed.


Non-Armored Cable flexible runs more than 6 feet need a ground wire.



said by nunya, at dslreports.com, +1'ed by Speedy Petey:


  Most BX is neither safe or acceptable for use as an equipment grounding conductor. Only BX which includes a bonding wire or bonding strip may be used as ground. Most "BX" style AC cable installed in the 30's-60's does not have the required strip.
  
  Using BX as a ground is extremely dangerous, because it is not a low impedance path to ground. Rather than clearing a fault, the spiral jacket (especially in places where it might have some corrosion) will heat up like a toaster element.
  I've witnessed this several times. The cable clamps in the original boxes also were not typically listed for use as a grounding electrode conductor.