Rear brakes lock up.

[Priesty]

I recently failed a road test for registration because my rear brakes lockup under firm braking. 
First report from repairer was that both rears locked up, and that he had inspected & re-adjusted shoes etc, but problem was still evident. I purchased an adjustable proportioning valve thinking I would fit it for now, just to pass my roadworthy test & then upgrade the whole brake system (including discs for the front) at a later date. However when I did a few test brakes myself, it appears it is only the right rear that is locking up, so obviously the adjustable valve will not be the solution, so looking for other clues to solve issue.
All wheel cylinders have been replaced & system bled & adjusted, handbrake works ok, drums are ok, shoes serviceable, no visible oil or fluid residue or leaks.
Anyone have any thoughts on possible causes ?

[crash340]

Mate, maybe check the short shoe is at the front, long to the rear (Ithink thats how it goes) mine were backwards

[brads70]

Try using the truck wheel cylinders. Stock bore is 15/16" and the trucks is 7/8" (Raybestos WC37236, DORMAN  W37236). 
http://www.cuda-challenger.com/cc/index.php?topic=66387.30
http://www.cuda-challenger.com/cc/index.php?topic=85620.0;nowap

[Fern]

The power brake booster may be bad.

[Priesty]

Mate, maybe check the short shoe is at the front, long to the rear (Ithink thats how it goes) mine were backwards

Yeah, first thing I looked at, both ok.
I'm gonna try & switch the drums, just to see if that makes the other side lock instead. I have a new replacement drum on the LH side only as original had oval holes, already backed off adjustment as far as I dare on the RH & still locking so nothing to lose I guess.
Also should have mentioned brakes are 4 wheel drums, unboosted.

[jimynick]

How are the shocks and ride height? If the car dives too much or too quickly, it can unload the rear and make it easier to lock a wheel as well. Just my 

[bc3j]

I use early 90's Dakota truck 13/15" wheel cylinders from one with 10" brakes.  Nine inch brakes have 7/8" wheel cylinders. I have no proportioning valve and have no rear lock up. Hope you get it fixed.

[Priesty]

Thanks all for your suggestions.
I ended up taking it to a local brake specialist in frustration. He said he went back to basics & re-adjusted all 4 corners & it now appears fine. He did say that one of the fronts LH I think had to be redone twice to get it right, so may have been a problem there. But at least I have my roadworthiness certificate now so I can finalise the registration process. The plan is for a disc conversion down the track anyway, so all good for now. As always on this car it's fix one thing & another pops up, flat battery & no charge output from battery now, so off to auto-elec for a new/reco alternator, fun,fun,fun. Hahaha.