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A while back I de-linked the brakes on my XJ and without trying it deemed my XJ front Master Cylinder unfit for the task of driving the 2 front calipers which I believe have 38mm pistons. After looking around I bought an 82 Virago XV920 front M/C as that bike has un-linked brakes and the front has dual calipers stock. I installed the 920 M/C on the XJ and it worked great! I might be wrong but I think the XJ M/C is 1/2" bore and the XV M/C is 9/16" bore.
Fast forward to me buying the XS. This one came with an aftermarket front M/C without a brake light switch. This is a deal breaker for me. I have installed different handlebars and and thought what I really need is sloped resovoir M/C or another XV M/C but I didn't want to drop another $100. I looked at my old XJ M/C and thought why not, can't hurt.
I installed the original XJ M/C on the XS and it really didn't work well. The M/C required a full travel pull to apply braking action that was really not up to the job. I got to thinking that the XS has 45mm front pistins and the XJ has 38mm pistons so I wondered what would happen if I swapped the 2 M/C between the 2 bikes?
As of yesterday I have the original XJ Master Cylinder back on the XJ with de-linked brakes driving 2 38mm pistons. Somewhere between 1/2 and 2/3 pull results in a near lock condition on the front brakes!
The XS is now sporting the XV920 Master Cylinder (originally intended for dual discs) driving the 2 45mm pistons. Somewhere between 1/2 and 2/3 pull results in a near lock condition on the front brakes!
With the theoretical $100 I saved I bought an XJ clutch perch so that my XS mirrors will point at the same direction as they are quite different between the XS and XJ mounts
It would seem to me that the biggest challenge with the XS front brakes is the dia of the pistons. I noticed all the newer bikes use much smaller, multi piston calipers. I can also note that I have become really good at using the vacuum bleeder.
John
Fast forward to me buying the XS. This one came with an aftermarket front M/C without a brake light switch. This is a deal breaker for me. I have installed different handlebars and and thought what I really need is sloped resovoir M/C or another XV M/C but I didn't want to drop another $100. I looked at my old XJ M/C and thought why not, can't hurt.
I installed the original XJ M/C on the XS and it really didn't work well. The M/C required a full travel pull to apply braking action that was really not up to the job. I got to thinking that the XS has 45mm front pistins and the XJ has 38mm pistons so I wondered what would happen if I swapped the 2 M/C between the 2 bikes?
As of yesterday I have the original XJ Master Cylinder back on the XJ with de-linked brakes driving 2 38mm pistons. Somewhere between 1/2 and 2/3 pull results in a near lock condition on the front brakes!
The XS is now sporting the XV920 Master Cylinder (originally intended for dual discs) driving the 2 45mm pistons. Somewhere between 1/2 and 2/3 pull results in a near lock condition on the front brakes!
With the theoretical $100 I saved I bought an XJ clutch perch so that my XS mirrors will point at the same direction as they are quite different between the XS and XJ mounts
It would seem to me that the biggest challenge with the XS front brakes is the dia of the pistons. I noticed all the newer bikes use much smaller, multi piston calipers. I can also note that I have become really good at using the vacuum bleeder.
John
But once I put on Stainless Steel lines, it worked fairly well, about 2/3's pull but I had brakes! 
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