"The reservoirs are different shapes, but maybe they fit onto each other's master cylinder ? Does anyone know if the original 1990 reservoir fits onto the 1993 master cyl ? "
Reservoirs are generally interchangeable but you don't need to change it. The level connector on the replacement M/C can be left unconnected if the plugs don't match.

Thanks Redd, not sure what the "level connector" is. Just wondering about the 2 supply/return holes with seals on the top of the M/C. They would need to be the same distance apart and not have different flanges/lips.
But seeing as you say they are "generally interchangeable" I'll go ahead and purchase the m/c. Hope it comes with new seals. If the reservoir doesn't fit, I'll send the Carlton Crew round to talk to you..  
 
cheers,

With the original aluminum Aisin 4 bolt M/C's that were fitted to MR2's, Supras and Land Cruisers I have found the seals and hole spacing is the same and have swapped some of them over OK.
I found the holding screw can be very tight and a good screwdriver is needed.
 
By 'generally interchangeable' I meant different reservoirs can be swapped OK but the connectors will be different. The connecting pipes may also foul the reservoir.
 
 
 
 
 
 
 
 

I installed a 1" master from a Previa van into my 1990. All I needed to do was swap over the reservoir to get it to work. I also have a 15/16th master from a Previa that I might use as the pedal for street driving is quite heavy. Fine for the track once you get heat in the brakes (semi track compound pads) but not going to do that for some time now. Both are aluminum and a direct swap for the cast steel 5/8ths. They are all non abs units.

What I do need is the 1992+ booster. I've installed a 1994 proportioning valve already so it's the last thing I need to do to swap from GEN1 to GEN3-5 brakes.

Ok, I fitted a 15/16" master. The plastic reservoir from the 7/8" needed minor work to fit the new one. Namely the plastic that surrounds the bolt hole fouled on a bit of the master cylinder. I had to carefully remove about .5mm from  a segment of the plastic circle, with a soldering iron. Then happy days. Bench bled, fitted, bled at calipers and bingo.
The pedal is hard and at the top. Car is not yet drivable, due to other changes(Led lights, new boost gauge in dash and soundproofing the cabin) but already I can see the brakes will be nice. Retained the orig fat booster and the ABS (jury still out) and stock rotors and calipers. Will give final thumbs up/down after driving.....
  Thanks to all for the helpful advice......

Ok finally on the road again and can report the brake pedal is higher as expected and desired from the slightly larger master cylinder. Stock 1991 calipers / rotors are not the most powerful in the world, but they work fine.