Finally had a DE on a sunny day last week, the Michelins were new, the driving was awesome. Hardest braking I do is at the end of the main straight, thought I'd stand on it just to see what was what, and my rears locked up first (didn't wait for the fronts at that point).

I'm using Carrera brakes all around from an '84 911, Pagid Blue's, 205's, 34psi hot; I've got the orig proportioning valve still in the circuit. Is this thing at all adjustable? Rebuildable? Should I replace it with a tweakable, and if so, with whose, and where is a good place to locate it? I think the old "tee" trick would make things worse, not better, right?

You've pretty much answered your own question.
If the rears are locking first, you need to adjust it out.

Adjustment of the stock dubber is possible, I hear.....but why dick around with it? A "T" may work, but if not ????

Adjustable valves are inexpensive....kind of a PITA to install.....about a 3 on the 1 to 10 scale. Gud winter project.

Which reminds me. I'm doing my first DE this next Friday. Gotta back off the rear bias a bit.

Gunna let Ulf out to play on a track? Have fun, I wish I could go...

Yuppers.
The last AX convinced me that I could use *some* work on heel & toe. Best place to experiment is the track, I figure. My tiars have never had a heat cycle
either. I'll just go out and toodle around.....try not to scare the crap outta myself. Some instructor should appreciate that.

Call me stoopid....

Most people do....but MY question would be...Have you checked your FRONT brakes?

Could you have a swollen set of brake lines? I know you siad you have a 911 fornt end....but have you changed the lines? OEM lines can swell in the interior and block flow.....

Just a guess....check the obvious first.

There are tons of things that can affect bias/lockup tendancies...if your struts/shocks or alignment is letting the nose "dive" an excessive amount, the rears will unweight a lot and want to lock. Also did you do a thorough bleeding job and are sure that the fronts are working 100%, as Mikey pointed out? Are you running the same pad compound all around? I found that I could get nearly perfect bias on my car with a more aggressive compound up front than in back; I'm running a "tee" with 911 "A" calipers & rotors up front and 914-4 front calipers & 914-6 rotors in back.

On the other hand, maybe the Carrera rear setup is just "too much" without a prop valve?