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A 13mm handle bar master with no other changes, converted my single disk, 1975 R90 from goofy bad to acceptable and much more pleasant to ride.

Since my hunt for the 13mm is still a goose chase, I added a vented/hole style rotor and rebuilt the caliper with hopes it will help. I will install tomorrow.
I started to wonder though... if there was such a thing, a softer viscosity brake fluid might help.
Or perhaps there could be some sort of a different response due to cable length or something?
humm... hopefully the rebuilt ATE will help. It needed it. There was some nasty gunk and a chewed up piston that I replaced with a good one from the spare parts stash.

jjwithers wrote: ↑Sun May 28, 2017 12:11 am Since my hunt for the 13mm is still a goose chase, I added a vented/hole style rotor and rebuilt the caliper with hopes it will help. I will install tomorrow.
I started to wonder though... if there was such a thing, a softer viscosity brake fluid might help.
Or perhaps there could be some sort of a different response due to cable length or something?
humm... hopefully the rebuilt ATE will help. It needed it. There was some nasty gunk and a chewed up piston that I replaced with a good one from the spare parts stash.

Sadly, the single disk caliper, coupled with the stock under-the-tank, at best will be inadequate, at least in modern terms of what an adequate brake system can be. About the best that can be done (that I know of) is to make sure that each component of the stock front wheel braking system is up to snuff. You can 'chip away' at the braking system, such as switching a handlebar master cylinder in lieu of that crappy under-the-tank master cylinder. The best I can offer is to spend-the-money and spend-the-effort. A steel braided brake line from the master cylinder to the caliper is a good move, and a must.

And keep an active eye out for that scarce (and pricey) dual disk R/H fork slider. (which is needed for a R/H ATE brake caliper in addition to what you have)

That being said, I'm not terribly concerned about taking my '76 R75/6 into harms way. But I do try to keep up with situational awareness, which will be your single most powerful tool for keeping safe.

Oh, and a steel braided brake line from the under-the-tank master cylinder would be a very good move toward improving braking power. (The brake line doesn't 'balloon' out as much as a stock brake line will.)

It could be worse. You could be trying to make do with a stock /5 drum brake on the front wheel.

Ken, trying not to be pessimistic in Oklahoma

I rebuilt the caliper with seals and a piston that is in much better shape, bled the brakes, installed a hole-rotor (which personally, i'm not sure if it makes that big of a difference).
I can say that the feeling is much better. Not as amazing as my twin disk set up on my other bike but acceptable now. Definitely not like grabbing a block of wood anymore.
Switching to a 13mm master, if i could find one, would have involved getting wider handlebars so it would fit with the gauge cluster and new brake lines, etc.. It was too tall of a task for this bike.
Or the other option would be to get another caliper on the right leg, split the brake lines, etc...

Thanks to everybody for all the help.