Modern front brake caliper.

[Dvsrd]

Excactly what I suggested. If using a modern caliper with stock 70s discs, get rid of unnecessary material and weight where it's not needed.
Even using the same calipers as stock RDs, TZ discs were drilled between the M6 bolts holding them to the carrier, like in the attached pic.

As for commonly available calipers, I definitely prefer Brembo over oem Yamaha blue/ gold spot.
Here's why:
Blue spot has 26 and 30 mm pistons, giving approx. 2476 square mm piston area.
Brembo P4 30/34 (40 mm mount hole spacing), has 30 and 34 mm pistons, and 3230 square mm.
Brembo P4/34 (65 mm mount hole spacing) has only 34 mm pistons, and 3632 square mm.
There is also a 4 pad variant of the P4/34, mainly designed for supermoto racing, and as a bolt on upgrade from 2 pad P4/34.
The numbers tell me that a P4/34 gives 50% more clamping force than a blue spot, for the same pressure. Or put differently, the same clamping force with only 67 percent of the pressure.
The two first versions of Brembo is widely available as used, and will give a neater installation if an adapter bracket is needed. As I see it, choosing a blue spot over a Brembo only makes sense if they bolt straight to your fork without an adapter bracket. 
  (well, maybe more 2 dollars.......)

[m in sc]

but you can run into clearance issues.  also, the blue or yellow dot in real world application works just as well on an rd as a brembo. have used all options. best option is a bigger modern disc 1st, period.

[oxford]

but you can run into clearance issues.

This is a problem on vintage bikes trying to use a modern opposed piston calipers.  I've seen pics where the inboard side of the caliper was milled to get spoke clearance.  Using some unknown calipers it would mean at the very least disassembly and careful measuring and possibly trashing a caliper in the process.

[vmx51]

Has anyone tried one of the reproduction calipers from Economy Cycle ??

[Dvsrd]

Has anyone tried one of the reproduction calipers from Economy Cycle ??

Yes, I have one on my 350A. Works great with a 13 mm Brembo MC and a single braided line from EC. I cannot say I have noticed any flex in the caliper, but some claim that TZ racers went back to cast iron calipers in some cases.

[Dvsrd]

but you can run into clearance issues.  also, the blue or yellow dot in real world application works just as well on an rd as a brembo. have used all options. best option is a bigger modern disc 1st, period.

Yes, clearance issues may dictate caliper choice, especially with spoke wheels. Larger discs help with that, as the spokes angle inwards as they get closer to the rim. The optimal RD brake could maybe be a single 300 mm narrow band disc with a matching caliper.

[Jspooner]

That is more than enough brake for an RD. It's coupled with a 13mm master from a 250 ninja.

[teazer]

Economy caliper is super light - especially when fitted with alloy pistons.  On a race bike it would flex/expand the same way as the TZ alloy caliper did but on our drag race TZ/RD it works fine with a thin disk.

[oxford]

Economy caliper is super light - especially when fitted with alloy pistons.

Yes they are much lighter than a stock caliper, I have a set mounted on my rd with alloy pistons in them.  Pistons came from Eric at oily pipes gang.

[Dvsrd]

That is more than enough brake for an RD. It's coupled with a 13mm master from a 250 ninja.

AP Lockheed type caliper by Grimeca? 38 mm pistons?
What would the advantage be over a stock or EC reproduction caliper?
And what size is that disc?

[Jspooner]

That is more than enough brake for an RD. It's coupled with a 13mm master from a 250 ninja.

AP Lockheed type caliper by Grimeca? 38 mm pistons?
What would the advantage be over a stock or EC reproduction caliper?
And what size is that disc?

I am not a brake expert by any means and I'm not saying there's any advantage except that I know it works and it works well and has excellent feel...........the disc is a modified XS disc thinned out to about 4.5mm and made to look like a TZ disc.

[Dvsrd]

I am not a brake expert by any means and I'm not saying there's any advantage except that I know it works and it works well and has excellent feel...........the disc is a modified XS disc thinned out to about 4.5mm and made to look like a TZ disc.

So a 298 mm disc from a 73-76 XS650?

[retaRD]

I'm using a Ducati 320mm monster rotor, silver dot caliper (aluminum pistons) and a Yamaha warrior master from a quad.  Stainless braided line.  Best brakes out of ALL my bikes.
I laced the wheel with an XS650 hub, which has a wider flange width than the RD, so I did shave a LITTLE off the back of the caliper to clear the spokes, but with an RD hub it would've probably cleared, since the rotor is 320mm.
I took a gamble on the warrior master, as the perfect size master for the surface area of the silver dot pistons didn't offer a lot of options.  I knew the warrior quad used two 2 piston calipers with pistons that weren't large like the RD, looked factory, and were cheap and plentiful.  Worked out perfectly.

But no, not the stock rotor.  Clearance wouldn't be possible and brake performance is hugely improved with a larger modern rotor.

[Dvsrd]

Nice setup RetaRD!
So the XS hub is wider than an RD hub from center to disc mounting flange?

Also, that specific Ducati/ Brembo disc looks great, but is heavier than most other 320 mm discs, because the hub/ carrier is steel, not aluminum.

[retaRD]

Nice setup RetaRD!
So the XS hub is wider than an RD hub from center to disc mounting flange?

Also, that specific Ducati/ Brembo disc looks great, but is heavier than most other 320 mm discs, because the hub/ carrier is steel, not aluminum.

Thanks.
From what I recall, no, the rotor mating surface is the same distance from center, as I used the same rotor spacers from my earlier FZR rotor/caliper setup I ran on a stock RD hub.  I would have to check and verify that though. 

I was referring to the spoke flanges being slightly wider, which with a 298mm rotor would not easily clear an XS hub, but 320mm does enough, with some very minor shaving of the caliper. 

True, the stock Duc rotor isn't the lightest, but now that I have the setup dialed in I can try to source and aluminum hat version of the rotor.  The 320mm Duc rotor is still much lighter than a stock RD rotor with all the benefits of larger size and floating.