Afaik, there is only one valve in the braking system- this is the 'brake compensator valve' mounted above and connected by a linkage to the rear axle - this valve can also be referred to as a 'brake pressure regulating or proportioning valve'. This valve has been discussed in various previous threads.
It's purpose is to prevent the rear wheels locking up under heavy braking. Under heavy braking, load is transferred onto the front wheels (front of car dips) and load is reduced on rear wheels (rear of car raises) - this reduction of load on the rear wheels might cause the rear wheels to lock up if the brake force being applied wasn't reduced.
The brake compensator valve (C.V.) is controlled by the linkage connecting it to the rear axle. The C.V. is normally held open when the car is sitting normally on its wheels or driving. But if heavy braking takes place, when the rear of the car rises, the linkage allows the C.V. to close, reducing the brake fluid pressure applying the rear brakes.
The internal valve in the C.V. is spring loaded to cause it to close. The linkage connected to the rear axle only opens and then holds open the valve. The C.V. and its operating linkage is adjustable by a slot in its mounting bracket - it normally doesn't need any re-adjustment in service. Iirc, you have a 'shop manual? - the adjustment procedure should be covered?
The reason for bleeding the brakes with the rear axle supported is to ensure the C.V. is held open (same as when the car is on its wheels).
O.K. Enough of me
explaining stuff...
What you presumably want to know is what to do next, especially as regards the compensator valve?
There are 2 metal pipes (hard lines) going to the compensator valve - one is the fluid feed from the master cylinder, the other is the output from the C.V. to the 5th brake hose and then on to the rear brakes via the 3 way union on the rear axle.
Loosen the fluid feed pipe at the compensator valve by a few turns, press the brake pedal slowly and fully a few times and check if fluid is coming out of the slackened union (this may take some time if air is trapped in the feed line) - if fluid is flowing, then tighten this union, now loosen the other pipe union at the compensator valve and again press the brake pedal a few times and see if you're also getting a flow of fluid. If not the valve inside the compensator valve has probably stuck or corroded in place.
If stuck, the valve will either have to be renewed or it can be be-built (afaik, seals are available). But, before removing the valve, you might pull back the rubber boot covering the linkage rod where it connects to the C.V. and see if you can free it off.
If you disconnect the linkage rod from the axle you can try operating the rod and check if the valve is being operated or is stuck/seized. (pushing up on the link to the axle should push the C.V. plunger inwards).
Some additional points :-
The C.V. usually seizes in the open position (assuming the car has been stored on its wheels) and therefore shouldn't affect brake bleeding.
When pressing the brake pedal as suggested above, you might find that the brake pedal doesn't travel fully to the floor, if this is the case, don't worry, it will be because you've already bled the air from the front brakes - and as you have dual circuit brakes, one brake circuit not working e.g no rear brakes, means the pedal will still operate the remaining circuit, so the pedal will travel 1/2 way (or a bit more) to the floor and then no further.
I've vacuum bled brakes in the past and found that sometimes it doesn't get all the air out. I've seen recommendations to vacuum bleed, then let car sit e.g. overnight and if you suspect there might still be some air trapped, do a quick conventional manual bleed (using the brake pedal) to clear any remaining air (any remaining air will rise to the top of the caliper, next to the bleed screw, overnight). Some people even tap the brake caliper with a soft face hammer to dislodge any little air bubbles stubbornly clinging to the inside of the caliper piston bore.
Afaik, some people 'gravity bleed' the rear brakes on the 124's - i.e. open the rear bleed screws and let gravity push the brake fluid to the rear brakes, it can take some time... - Just don't let the reservoir run low or you'll have to start again.
P.S. Well done on your progress so far and happy to hear you managed to fit those pesky brake caliper lock plates with ease
