Well we'll have to wait and see what the actual outcome is I suppose; but, the OP originally reported no problem getting sufficient pedal and pressure prior to the problem with his not being able to seal the bleeder to be leak free. Then he mentions leaving the bleeder out and the system open for a period of time. Doing that will allow all the fluid to drain out of that caliper's vacant bleeder screw opening. Then later the OP details he applied vacuum to the bleeder and could not draw any fluid, if there was fluid in the rear reservoir he would have drawn it towards the caliper.

All those things considered I'll stick with the drained rear reservoir compartment until more information pointing to some actual MC malfunction is revealed.

The good thing is it will be quite easy and inexpensive for my theory to be confirmed or refuted whereas fooling with the MC will require much more effort and expense to verify the theories around that.

I agree the old unit needed replacement due to lack of use and age..

but your description of pedal travel going to full stroke to allow the other circuit to operate is not correct... both front and rear are operating independently and at the same time...

There can be differences built into the system due to the amount of fluid required for regular brakes on the rear and discs on the front.. but those are adjusted by things like bore size...so the same pedal travel stroke produces those needed differences....

https://www.google.com/search?q=master+cylinder+dual+circuit&biw=939&bih=602&tbm=isch&imgil=A-S_pFUtkEZ4KM%253A%253B8nCRTg4G0hK-KM%253Bhttp%25253A%25252F%25252Fwww.tegger.com%25252Fhondafaq%25252Fmastercylinderreplace%25252Fhoww orks.html&source=iu&pf=m&fir=A-S_pFUtkEZ4KM%253A%252C8nCRTg4G0hK-KM%252C_&usg=__qhrgALcZGJRLw088q7PD1cSSc0I%3D&dpr=1.09&ved=0ahUKEwjv7IWG2MfQAhXnilQKHS_XCHYQyjcIbw&e i=1Ck6WO-yCeeV0gKvrqOwBw#imgrc=A-S_pFUtkEZ4KM%3A

You have miss quoted me and have absolutely no idea how a single bore dual circuit brake master cylinder operates.

Try reading what I posted again without cutting relevant information out of the middle.

I did not misquote you because I did not change anything you said... I just left out the part which did not pertain to what I was pointing out was not true.

Did YOU go and look at the pics in the google article ?

This statement "pedal travel will go full stroke to allow the other circuit to operate."
is not true... both the front and rear operate upon the same stroke. One is not having to travel full stroke... ' to allow ' the other to operate.

I agree with this. I prefer the simplicity of the old two person brake bleeding process. I don't see a healthy mc being damaged by this process.

I've highlighted the portion of my quote you deleted and colored red the portion vital to the point I was making.

97 SL320 One circuit developing a leak has nothing to do with the functioning of the other section of the circuit. It does not have to do anything to ' react ' to the leak in the other side... it is oblivious to that leak.. it just functions according to the physics incorporated in how it was constructed.... a bore with a sealing piston pushing brake fluid...

Losing fluid in one circuit doesn't force the other to go to full stroke to allow braking. The pushrods are a solid connection and will operate regardless of the fluid level.

What WILL happen is a dramatic change in FEEL of the brake pedal, making it far mushier (is that a word?) since you're now only compressing half of the working fluid. This does allow you to push harder on the pedal, but it isn't the result of pushing one of the circuits through full stroke to operate the other.

You should buy a pressure bleeder. I have one and it has allowed me to bleed my brakes all by myself. Also, you can do a brake fluid flush to get rid of all the old brake fluid. FYI, you should also change out all 4 brake lines because they do break down internally from old brake fluid, it's cheap insurance from your brakes locking up and not releasing.

I agree... which particularly pressure bleeder did you buy ? which you are happy with.. OR would you get a different model had you known about the advantages in the designs ?

I got the Motive one from Amazon with the European adapter. I actually fill the master cylinder to the very top with brake fluid instead of filling the Motive pressure bleeder, there is no cleanup after you are done! Also, you can purchase other master cylinder caps for your pressure bleeder to work on other vehicle makes.
I should have bought the Motive pressure bleeder 20 yrs ago when I first started working on cars!!!!

The push rod is only attached to the left piston, a column of fluid operates the right piston as there isn't a rigid connection between the two. If there was a rigid connection, the braking system half that had the least free travel would cause the piston to stop moving before the other had a chance to build sufficient pressure.

The pic referenced in LMs post shows a spring between the two pistons, this is merely one of two return springs.

When the column of fluid is lost between the two pistons due to a leak ( left brake line ) , the left piston must travel until it contacts the safety stop on the right piston. At this point the right piston is moved to generate hydraulic pressure. The pic referenced by LM makes note of this " This part begins to compress."

If the leak occurs on the right piston, this right piston must bottom out in the bore before the left piston can generate any pressure.

In either case, a leak will allow one of the pistons to travel full stroke.

You looked at the wrong picture.

http://www.gomog.com/allmorgan/MC1.jpg

I also fill the master cylinder to the top but did not spending $ buying a Motive. I took a spare cap, drilled a hole and put a tire valve (found strewn all over the ground at tire shops) in the hole. Apply compressed air with a hand bicycle pump and that's my pressure bleeder setup. No cleanup other than wiping down the tire valve and cap when done.

It's interesting how LM says I'm wrong and provides a web site that proves what I'm saying is correct. Still want to go another round?

MASTERCYLINDER

For the complete story with pics that matter, use the link provided. ( The MC in the pic does not have the 2 return springs, apparently they have been omitted for clarity. )