It is my understanding with a dual master cylinder, it is designed such that if you have a leak in the front brakes that the rear brakes should still work and like wise, if you have a leak it the rear brakes, the front brakes should still work. Here is a good article that I found that describes the internals of the tandem/dual master cylinder and how they are supposed to work when a leak occurs: http://www.britishv8.org/articles/ta...-cylinders.htm

What I'm finding with the hydroboost power brake setup I have in my MK4 is if I have a leak in either the front OR the rear hydraulic brake circuit, then I loose both front and rear brakes. This is the test I ran. I bled both the front and rears and had good pedal pressure and good brakes on all 4 wheels. I had my son apply the brakes (pedal went down about 1/3 of the way) and I went to all 4 wheels (car was on jacks) and confirmed that all 4 wheels had brakes. I then cracked the bleed on the front right caliper and the pedal went to the floor and my son held it there. I then went to all 4 wheels and I could turn all 4 wheels, no brakes front or rear. I repeated this test but this time I cracked the bleed on the rear right caliper and same thing, lost brakes to all 4 wheels.

What I believe is happening is with the hydroboost/master cylinder setup I have, there is not sufficient pedal / push rod travel to cause the master cylinder internal pistons to "bottom out" and allow the good brake circuit to pressure up (see article). I measured the total linear movement of the push rod coming out of the hydroboost that pushes on the master cylinder and it was about 0.9" with full pedal travel. I have the Wilwood brake pedal box and the pedal is as far forward as it will go (almost touching the 3/4" tube) so I have the maximum amount of travel possible. I also confirmed that there is no slop between the pushrod and the master cylinder when the pedal is in the rest position. On the bench, you could push in on the piston of the dual master cylinder and it will move about 1.5" before it bottoms out which seems to indicate to me that I might need closer to a 1.5" travel on the push rod vs the 0.9" that I'm seeing.

So....... has anyone experienced this problem? Does it sound like insufficient pedal travel is causing my problem? Any way to solve this problem?