Gonna try to make this a TL;DR post since I have a bad habit of turning all my posts into 10 page essays.

My brakes are all buttoned up, and I'm fairly certain I have a bad master cylinder. Just want to be 100% sure before I order a new one. Every time I push the brake pedal, the brakes bite upon pushing, but the pedal falls to the floor as well. Once the pedal hits the floor brake pressure is lost. The master cylinder has been bench bled, the lines have also been both vacuum as well as pump bled. Can't get the pedal to hold stiff without falling to the floor.

One other thing I noticed is that when I vacuum bleed the brakes, air bubbles literally never stop coming through the lines. I'm used to seeing solid stream of fluid through a vacuum bleeder after 1-2, maybe 3 cycles around the car. I've gone through almost 3 1 liter bottles of fluid and I'm still getting air in the vacuum bleeder, as well as the pedal falling to the floor.

 

Unfortunately we can't help you be 100% sure either. All I can tell you is I chased a bad pedal for over a year. I went through 2-3 master cylinders, lines, had all the calipers rebuilt, and still nothing. What finally solved my problem was to have the system bled with a professional vacuum. I know you said you've already tried vacuum bleeding, but I'm willing to guess its one of those Mittyvac DIY style bleeders. Take it to a shop with a true, roll around based bleeder. Apparently a lot of air gets trapped in the 4-pot calipers and is difficult to get out by traditional methods.

 

I've never found our calipers particularly difficult to bleed, I suspect your initial call of bad master cylinder is correct.

 

I'm going to go after it some more tomorrow. I am pretty sure the master is bad. It was included with the purchase of my 5-lug and brake setup. Was kind of an after-thought when I was putting everything else together, and is a tad rusty. Honestly looks like its seen better days. I know for a fact my old master was bad because the single piston brakes worked for a while, then started doing this same thing all of the sudden. That's the whole reason I did this conversion is so that my brakes would work again, and the upgrade to 4-piston is nice. Seeing as how a new T2 master is only $40, and just a pressurized bleed and flush at a local shop is $75, I think I'll take my chances with a new master cylinder first. Just finding it ironic that after replacing every piece of braking equipment (exception being the hardlines) I'm no better off then I was before. Still cant stop my car without dropping gears like an idiot...

 

So if your master was bad, a lot of times the brake fluid gets on the brake booster diaphragm, which should be changed, but that had nothing to do with air in the system.

Another thing I would do is clean everything off and check for any leaks anywhere else. Our cars are old and every component of the braking system should not be overlooked. Are you buying new, or reliable remanufactured parts?

I try to save money here and there, but I see a lot of people getting used brake master cyl, calipers, lines, etc. Especially if it's an easy pick part junkyard piece. Even good used brake parts that have been sitting in some garage for a period of time.

If I ever put used parts on my car, I usually rebuild them, and if you do not have the proper knowledge and tools to do it yourself, you should have an experienced person help you.

sorry if i sound like in attacking you, I'm not. I'm mentioning this because I see it all over the forum. If you did happen to get used calipers, I would disassemble, inspect, then rebuild if possible

 

I completely understand man. I bought all this conversion from a forum member. The calipers and hubs as well as the booster are all in good shape.the master cylinder was the only thing that looked old. I'm certainly no expert, but I have the knowledge for a job of this calibre.

Also sort of an update. Walked into the garage to look at things today, and I'm now almost positive my master cylinder is bad, as there is some fluid leaking down the front of the booster where the master cylinder mounts to it.