Based on this thread:
http://www.thegsresources.com/_forum/showthread.php?t=204208
It looks like he has an 80 GS1000G. It really helps to put one's bike in one's signature...
Alpha-sports.com shows both reservoirs for the GS1000GT still being available new OEM. G&S says about $16 for the front (reservoir only) and $37 for the rear (reservoir kit, includes cap).
Alpha-sports's cross-reference says the rear was used on:
1979 SUZUKI GS1000LN - REAR MASTER CYLINDER
1979 SUZUKI GS1000SN - REAR MASTER CYLINDER
1979 SUZUKI GS550LN - REAR MASTER CYLINDER
1979 SUZUKI GS750EN - REAR MASTER CYLINDER (GS750C,EC,N,EN)
1979 SUZUKI GS750LN - REAR MASTER CYLINDER
1979 SUZUKI GS750N - REAR MASTER CYLINDER (GS750C,EC,N,EN)
1979 SUZUKI GS850GN - REAR MASTER CYLINDER
1980 GS850GL - REAR MASTER CYLINDER (MODEL T)
1980 SUZUKI GS1000ET - REAR MASTER CYLINDER
1980 SUZUKI GS1000GLT - REAR MASTER CYLINDER (MODEL T)
1980 SUZUKI GS1000GT - REAR MASTER CYLINDER
1980 SUZUKI GS1000ST - REAR MASTER CYLINDER
1980 SUZUKI GS1100LT - REAR MASTER CYLINDER
1980 SUZUKI GS550LT - REAR MASTER CYLINDER
1980 SUZUKI GS550T & ET - REAR MASTER CYLINDER (FOR CAST WHEELS)
1980 SUZUKI GS750ET - REAR MASTER CYLINDER
1980 SUZUKI GS750LT - REAR MASTER CYLINDER
1980 SUZUKI GS850GT - REAR MASTER CYLINDER
1981 SUZUKI GS1000GLX - REAR MASTER CYLINDER (MODEL X)
1981 SUZUKI GS1000GX - REAR MASTER CYLINDER
1981 SUZUKI GS1100EX - REAR MASTER CYLINDER (GS1100EX)
1981 SUZUKI GS750EX - REAR MASTER CYLINDER
1981 SUZUKI GS750LX - REAR MASTER CYLINDER
1981 SUZUKI GS850GLX - REAR MASTER CYLINDER (MODEL X)
1981 SUZUKI GS850GX - REAR MASTER CYLINDER
1982 GS1100E,ES - REAR MASTER CYLINDER
1982 GS1100G - REAR MASTER CYLINDER
1982 SUZUKI GS1100EZ & GS1100ESZ - REAR MASTER CYLINDER
1982 SUZUKI GS1100GKZ (Touring) - REAR MASTER CYLINDER
1982 SUZUKI GS1100GLZ - REAR MASTER CYLINDER
1982 SUZUKI GS750EZ - REAR MASTER CYLINDER
1982 SUZUKI GS850GLZ - REAR MASTER CYLINDER
1982 SUZUKI GS850GZ - REAR MASTER CYLINDER
1983 GS1100E & ES - REAR MASTER CYLINDER
1983 SUZUKI GS1100ED & GS1100ESD - REAR MASTER CYLINDER
1983 SUZUKI GS1100GD - REAR MASTER CYLINDER
1983 SUZUKI GS1100GKD - REAR MASTER CYLINDER
1983 SUZUKI GS1100GLD - REAR MASTER CYLINDER
1983 SUZUKI GS850GD - REAR MASTER CYLINDER
1983 SUZUKI GS850GLD - REAR MASTER CYLINDER
1984 SUZUKI GS1100GKE Touring - REAR MASTER CYLINDER
And the front was used on:
1980 SUZUKI GN400XT - MASTER CYLINDER (GN400XT, GN400XX)
1980 SUZUKI GS1000GT - FRONT MASTER CYLINDER
1980 SUZUKI GS450ET - FRONT MASTER CYLINDER
1980 SUZUKI GS450ST - FRONT MASTER CYLINDER
1980 SUZUKI GS750ET - FRONT MASTER CYLINDER
1980 SUZUKI GS850GT - FRONT MASTER CYLINDER
1980 SUZUKI GS850GT - FRONT MASTER CYLINDER
1981 SUZUKI GN400XX - MASTER CYLINDER (GN400XT, GN400XX)
1981 SUZUKI GS1000GX - FRONT MASTER CYLINDER
1981 SUZUKI GS450EX - FRONT MASTER CYLINDER
1981 SUZUKI GS450SX - FRONT MASTER CYLINDER
1981 SUZUKI GS750EX - FRONT MASTER CYLINDER
1981 SUZUKI GS850GX - FRONT MASTER CYLINDER
1981 SUZUKI GS850GX - FRONT MASTER CYLINDER
1982 SUZUKI GS450EZ - FRONT MASTER CYLINDER
1983 SUZUKI GS450ED - FRONT MASTER CYLINDER