M.C.
Mine is identical. Lots of time the large reservoir is the one that goes away. If this is to the rear of the master cylinder, then the fluid can leak out, into the master vac, and when you start the car it gets sucked into the engine and combusted...
The large reservoir is for the Front brakes ALWAYS, as the discs require more volume to actuate than the drums do. THe big reservoir is for the front, that's the theing to rmember.
They are correct in saying some have front/front layout of the master cylinder, some don't. I have cars with both. On the one where the drum section (small reservoir) is in the rear, IT leaked into the master vac regularly. Drove me nuts because the wheel cups out back were consistently dry!
I think there is somehow the vacuum in the Master Vac can leak towards the master cylinder and work majic on the master cylinder sealcup assemblies in the nearest portion of the thing. Just my theory, but I've never had the front half of the master cylinder fail.....EVER. Not on a Dutsun.