Chrysler Repair: brakes and servo, brake pipes, vacuum pipe

Question
QUESTION: Hi Roland,
Vehicle: LHD 1993 Voyager SE 2,5 litre 4cylinder.  
I started losing brake fluid and it was getting worse.  I had the car checked out by a local (generalist) garage (I am away from home, my tools and workshop).  He found that the brake pipes (front to back) were corroding and weeping.  He replaced them.  It seemed OK for a few days but then the brakes were going to the floor and the warning light was coming on.  With use and pumping they seemed to pick up.  I took it back to the garage today and the brakes were much worse and not pumping up and only just working.  The garage found that the rear "compensating valve"? was weeping and suggested that air was being drawn in.  I am not convinced. I ordered the spare part (at great expense)and went to get my car back while I wait.
On collecting the car, the brakes were still awful but i noticed that, when I turned off the engine, I could pump up the brakes.  I disconnected the servo vacuum pipe and the brakes are solid with no warning lights.  I suspect the servo but can't see how it would produce these symptoms.

Help greatly appreciated,
Regards,
Roger

ANSWER: Let me check the manual and get right back with an answer.

---------- FOLLOW-UP ----------

QUESTION: Hi again Roland,
A bit more info.  I have just had a test run with servo disconnected and when I push really hard on the brakes, the warning light does come on.   Obviously, I can't exert as much pressure as the servo and I can't floor the pedal the way the servo does.  Sorry if I am confusing the issue.

Roger

Answer
The warning lamp can be caused by either low fluid in the master cylinder reservoir OR by an imbalance between the two hydraulic systems due to a leak on one side of the system (proportioning valve is where this type of issue would be detected). That imbalance may be a leaky proportioning valve, or brake line fitting or a leak at one of the wheel cylinders. If you are loosing fluid then the source of the leak needs to be determined.
I would most appreciate your rate/nomination of my answers.
Roland