Shimming BM57 Proportioning Valve

Morning all, another battling with brake balance thread...

I have a Nabco branded BM57 installed by one of the previous owners - I'm not sure if it's R33 GTR, or 300ZX, or whether it makes any difference. My aim is to shim the internal proportioning valve to allow me to override it and control it via an external proportioning valve instead. This seemed like a fairly simple procedure, until I opened it up and got to this:

https://live.staticflickr.com/65535/...77cf379e_b.jpg20201216_215326 by DaveyBoyDave, on Flickr

It looks like the red anodised part needs to be unthreaded to get to the spring and valve underneath, but there is no way I can see to do this and I'm quickly coming to the conclusion that it isn't possible with this variant of BMC.

So, couple of questions:

1. Has anyone got experience of successfully shimming the valves on different BM57s? Perhaps Tokico models? It seems to be possible on BM50s but I'd rather keep a BM57 if I can. There seem to be different flavours of OEM BM57 around - some with a open vent port in the side (mine doesn't), different shape reservoirs, different colour caps for example. But it's going to be expensive business just trying different types at random!

2. Failing that, can those that have successfully shimmed BM50s confirm which ones do work - e.g. Nabco or Tokico, certain part numbers. Is there any difference between S14, non-turbo Z32 or any other variants?

Help!

For future reference, should someone be searching for the same issue, I can confirm:

The two Nissan BMC OEMs, Nabco and Tokico, whilst externally similar and interchangeable, use different components for the internal proportioning valve.

My Nabco BM57 came with my car so I'm not sure whether it came from a Skyline variant or Z32 (or even whether they are different), and I don't think you can remove the bits of the proportioning valve to adjust it (see photo above).

I just got hold of a second hand Tokico BM57, purportedly from a Z32, I can confirm that this type of proportioning valve can be disassembled and shimmed to override it:

https://live.staticflickr.com/65535/...569ff76b_b.jpg20201223_130343 by DaveyBoyDave, on Flickr

There may be other options as well, which I briefly investigated:

HFM make new BM57s, but weren't really sure if the prop valve is shimmable. I was somewhat fobbed off by their customer support, but some more sustained investigation might yield a more positive result.

The BMC from a Y60 Nissan Patrol has no internal proportioning valve, avoiding the issue altogether - Part no. 46010-02J00. One of the ports is on the top, however, and they are fairly hard to come by, it seems. Beware, this generation Patrol had drum and disc rear brake options; the former comes with a 1" (BM50) BMC, the latter a 17/16 (BM57).