if I understand correctly, if a master cylinder fails whether dual or single, diagonal or front/rear, the entire system fails anyway?
Yes, you are understanding me correctly here. The idea of a Tandem System is supposed to be fail safe (in the event of a hydraulic failure), however with the Green, Black (you have one of these types) and Yellow Tag masters, I have simulated failures in these and with all, in a failure mode, the pedal goes all the way to the floor with no pressure there what so ever. A failure is very easy to simulate, just crack open a bleed nipple on either front or rear.
The other 'thing' with Tandem systems, is there's more rubber in the system so the pedal has a marked spongier feel when compared to single circuit.
Is a pressure valve still necessary if I have 5/8 rear wheel cylinders and 7.5 disks?
Yes. The Valve is there to stop the Rear Brakes from locking up.
In production, there was 3 different valves used. There was that mostly used with the Single Circuit set up, that fits to the front rail of the rear subframe, there was also in some cars the Inertia Valve (my favorite), which was mounted to the right rail of the rear subframe. Either of these are good, though I rate the Inertia Valve the cream of the crop as it takes in to account how fast you are stopping and the load in the back when controlling the braking pressure to the rear brakes, rather than reaching a certain pressure and shutting off regardless. The third valve is the FAM7821 and it's 5 port version FAM4670 are in my experience, little more than junk. They are overly complex for what they do, add even more rubber in to the system, part of the 'polished' internals breaths to atmosphere, only leading to premature rusting, they can't be dismantled for servicing and there's no kits available in any case.
If so, what is the most reliable option? None are cheap but safety is always priority. I think I read somewhere on the forum or it was someone on a local chat group who suggested the use of a manual pressure valve at the rear split on a front/rear system instead of a 3way connector.
From my experience, I don't think any one particular set up (single vs tandem) is any more reliable or unreliable than any others, except for those that have the FAM7821 and the FAM4670 valves in them, as noted above. The best thing you can do with any brake system to be sure of it's reliability is regular maintenance, including changing of the fluid, a full close inspection of all the lines & hoses and periodically stripping down all cylinders giving them a light hone and rebuilding them with new rubbers. The factory recommend the latter here every 3 years of 40 000 moles, which ever is sooner.
In regards to manual pressure valve - is this a manually operated valve or a (manually) set pressure point ?
I would suggest not running a manually operated valve, if that's what it is.
You can run with a manually adjusted set point type to really optimise the rear brakes, just be aware, they can take a while to set up and should be done in worst conditions. In the pre-set factory versions, there was (from memory) 5 or 7 different pressure valve used.
