Here in NZ from the late 70's onward minis were fitted with a dual circuit master cylinder, and a "PDWA" (link below) in the bulkhead. Theoretically the PWDA contains a plunger that can move side to side to equalise pressure between front and rear circuits.

(later version)http://www.minispares.com/search/classic/pwda.aspx

From what I can tell the secondary function of firing a warning light if the pressures were too far unequal was not used.

Anywho, in my last car I was running a dual circuit MC plumbed directly to front and back. Correct cooper 's'/GT rear wheel cylinders. Never had any problem with rear lock-up or pedal travel etc. From memory car stopped very very well.

I am now trying to sort a car with dual circuit MC and PDWA fitted - having a helluva job shifting the last of the air out and I'm tempted to bin the PWDA and plumb direct again. Fronts are minispares 4 pot alloy callipers, rears are correct cylinders for discs, no booster. Anyone else running same setup?

I expect pedal to be a little longer than 2 pot fronts, but other than that, anyone forsee any issues? I'm inclined to think that PWDA doesn't do a damn thing other than create an air hidey hole.

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http://www.mini-hb.co.nz

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SPO41 Mk2 Cooper S very original
1993 R32 Nissan Skyline GTR

The PDWA valve doesn't equilise pressure. The line for the front brakes doesn't actually do anything - it's just a splitter. The plunger inside is for the rear brakes, and that is used to restrict fluid when the pressure is too high. This stops the back brakes from locking up under heavy braking. I had issues with the PDWA valve and I reconditioned it (just replaced the o-rings reall), and that restored the pedal height issues I was having.

If you can, get a bolt the same size as the brake unions, and wind that into the outlet of the master cylinder for the rear brakes. If that fixes it, re-attach the MC to the valve, bleed it and put the bolt in the valve outlet. If the pedal is firm when it's in the MC but low when in the valve, then it's the valve you need to fix.

The second paragraph makes sense to me, about isolating where the issue is, however you've lost me with the first one. You're definitely talking about the cylinder attached to the firewall in the engine bay below the MC, and not the valve bolted to the rear subframe?

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http://www.mini-hb.co.nz

I'm talking about what is often called a PDWA valve found on late Australian cars - looks like this:

I've just read through the description of that unit you provided the link for, and it is different to the one I have experience with. At the bottom it says '5 port type of this on Australian cars is no longer available' - that's the one I'm used to. Have a look at the two links below:
Late Australian Type
UK Type

To clear things up, the Australian ones (and the UK ones too, I imagine) are not true PDWA valves. PDWA stands for Pressure Differential Warning Actuator, and they are used where a dual circuit system is used, and compares the pressure of each circuit. If one circuit drops in pressure, they can activate a light on the dash. On late Australian minis, this was the 'Brake Fail' light found on the dash. The valves here do not do that - there is no sensor or wire that runs to the dashboard. There was a PDWA valve fitted to late Australian models, but this valve is not it - the PDWA valve was integrated into the late dual-circuit master cylinders, and most have seized up and stopped working by now. You cvan tel a valve is not a PDWA valve if it has no electric plug or wires - it can't give a warning without those!

If yours is not a 5-port valve, then the info I provided before may not be 100%, so sorry if that has caused an confusion. But if this operates anything like the 5-way one, the valve does nothing for the front brakes, and only limits the pressure that can be applied to the rear brakes. I have heard that the valves bolted to the rear subframe do work as an alternative, but for the Australian models using the valve is easier, as the valve also acts as a splitter. If you have the 4-port valve, I don't know of any reason it would be better than the rear subframe version.

OK we're talking about the same thing. I did see mention of the Aussie version, but didn't think to look on an aussie parts supply site for more info!

You're right they're not a true PDWA, as in they do not warn (a light). However I think the other purpose is still true - when pressure is unequal the internal piston shifts, thereby equalising pressure. All theory of course - in my experience, and mentioned by others in the forum, if one line fails (a split line, or simulated by opening a bleed nipple) the pedal simply goes to the floor as there is not enough MC pedal travel to compensate for the piston's movement. Kinda like they had a good idea, but didn't quite follow through to execution. Maybe it works better in other BMC cars with different MC or pedal setup.

However, I did use the other info as a reminder and isolated the circuits. I confirmed what I suspected, that it was the rear line with air. I've got a better pedal now, and with help from a mate should be able to get it sorted. Thanks for your help.

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http://www.mini-hb.co.nz

FYI, when I fitted the current yellow tag master cylinder to my 1978 mini I also fitted the older type pressure limiting valve to the rear subframe. I also have fitted Minisport 4 pot front brakes, 8.4 size and new rear wheel cylinders. I only had issue with the bleed nipples on the front calipers which we're leaking ever so slightly, but fixed now by replacing with new ones. I did replace all brake lines with new ones before refitting the sub frames so it was easy to rework the lines to suit, including the correct size nuts for the new master cylinder.
Brakes are great with no booster fitted.
Good luck

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

I currently have the 5-way valve with a yellow tag MC, and 7.5 cooper brakes on the front. The pedal is quite firm, but I don't mind that - all comes down to preference I suppose.