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looking at the Omega, most cars have a master cylinder with two different bore sizes, 25 and 20mm. The 25mm part feeds both front wheels, whilst the 20mm part feeds both rear wheels.
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Hmm. I'd never realised the two circuits had different master cylinder bore diameters. You learn something every day. 
Kevin
I think yours will have equal 25mm brake master cylinder. As far as I can tell, they changed sometime after facelift, but before 2.6/3.2.
the size is cast into the body of the master cylinder on the top, below the reservoir.
there's also names on the BAS block for the circuits, but they're in German. HR, for example is Hinten (rear) rechts (right).
just looked it up from Bosch - HR = hinten rechts (rear right), HL = hinten links (rear left), VR = vier rechts (front right) and VL = vier links (front left)
mind you, if there's any German speakers on here, I'll be embarrased, 'cos the German above is a complete guess.