OK, you are faced with the job of replacing one or both of you master cylinders on your chrome bumper MGB.  You start looking at and realize that both M/C's are held in place by two bolts with nuts.  Both of the top bolts/nuts and fairly easy to get too, but the two on the bottom of the M/C's look to be a real pain.  You cannot get to them to be able to put a wrench on either, and sometimes there just is not enough clearance between the nut and the M/C to get a wrench on even if you could get to it. 

        I normally pull the whole pedal box so I can remove these while on the bench. Pulling the pedal box is pretty straight forward, so I will not go into that.  BUT, I can tell you how to avoid this problem next time.

         Before you replace the M/Cs, take a few minutes to thread the mounting holes with a 3/8" X16TPI tap.  There is no need to drill the holes because they are almost the correct size to thread, already.  Just carefully run a tap through the holes, making sure that you produce good straight threads.  Once you have the holes threaded, you can simply install the M/Cs using 3/8" X16TPI bolts about 1/2" long, with lock washers, from the front side.  Next time you have to remove either or both, you just remove the bolts from the front.  In fact, you can leave the bolts loose to aid you in reconnecting the hard lines.  This alone might prevent you in cross threading the hydraulic fittings and prevent a fluid leak.