Faulty master cylinder ?

[DaveMarcotte]

Just installed new master cylinder and new Prima front brakes. Front brakes will not bleed! I'm using a vac pump which I've done in the past. Tried the rear brakes and they bleed.
To test, I disconnect one of the front lines at the master and air suction from the vac pump was there put not sucking fluid from the res? I also disconnect line from res and fluid flowed. Does this sound like a bad master cylinder or I'm I missing something? Thanks in advance.

[wetminkey]

I'm afraid that I've also learned the hard way, because of the same experience,...
Master cylinders must be bench bled prior to installation. And I've never seen instructions to do it first.
I've seen this same problem posted a couple of times, on the forum, in the last few months.
Think "vapor lock",...The master must have fresh brake fluid pumped through it (from, and to, both reservoirs) prior to installation. Damn messy task, but it will break the airlock that keeps you from bleeding your brakes right now.
Best of luck with your Spider!
Todd.

[RRoller123]

I am confused by all this, because I bought a new MC from AR last year, bolted it in, added new lines, hooked everything up, filled the dual reservoir, opened the bleed screws at each caliper and gravity bled the entire system with no issues at all. Odd.

After giving this a little thought and jogging my memory, I filled the MC after bolting it on, from the top, through the 3 brake line mounting holes, then attached the lines, filled the reservoir, etc etc. Maybe this makes a big difference?

[DaveMarcotte]

Two years back I installed a new master. The adjustment nut on the booster was out too much (found that out on test drive. Brakes where engaged). Point is, at that time I could bleed the whole system with vac pump. This time, can only bleed the back but the fronts would only bleed with zero pressure from the nut? Which isn't enough to operate the brakes. Called Vick's, couldn't offer a reason other that the new master cylinder (although extremely rare) is faulty.

[wetminkey]

I have allowed a Mustang brake system enough time to gravity bleed itself from the master, but that's a very long process!
Over the years (and after installing a good number of new brake master cylinders) I've found that it's easiest to just fill and bleed the master first, rather than have to remove it and go through the process later. The master's internal pistons have very small holes and breaking surface tension in these seems to be a problem, even with a vacuum bleeder.
Some will bleed, some won't,...take your chances!
Todd.

[pantera928]

When I worked on these for a living, we just installed the master cylinder dry and bled the master first via the 3 lines that come from it going to the calipers and then bled the rest of the system. But, that is just my experience. It certainly wont hurt to prime it on a bench though.
FYI, it you do not have a 10mm crows foot and an extension for your ratchet, those line are a pain in the butt to loosen and tighten..

[DaveMarcotte]

My problem isn't that the front brakes won't bleed. It's that they only bleed when there is zero pressure from the push rod on the booster. Ordered a new new master

[DaveMarcotte]

So it wasn't the new master. The push rod had to be adjusted back. Brake still seem a little spongy lots of Peddle travel. Will bleed a third time. Cheers.
Another round of bleeding with vac, peddle bumps and tapping on the master cylinder. Worked like a charm.