I am so tired of this F'ing car. 1981.
New Lonestar Master Cylinder ($400). Unbelievable pain in the *** getting the brake line aligned to the back port and threaded in without fluid leaking out. (No room to maneuver) (Why didn't they bring the lines in to the MC on the engine side, instead of up against the firewall?). But eventually got it.
Now, I see fluid on the ground. I check and find fluid coming from the back of the Master at the Booster. Not a lot, but enough to concern me that it will back into the Booster and then I will have to go through the hell of replacing the Booster.
I measured the rod with one of those H shaped gauges and clearance was perfect.
Please, someone tell me it is possibly just residual fluid and I won't have to go through the MC swap process again?
I have over $30K into this money pit. Worst engineered cars that I have ever worked on and I have been working on Chevys for 50 years. Engineers/designers obviously never had to work on them.
Sorry for the mostly rant, but hoping someone can suggest something that I might try.

 

Oh Ed, settle down now.
Things will work out eventually.

I don't know how you can tell if that is old DOT3 or fresh. I think I would remove the master, give a good cleaning, remount.

Brand new master and it's dripping out the back? H-m-m-m-m-.
Me thinks you might have ruined the seals inside when you bench bled it.

How far did you shove the piston into the master?
A 1 & 3/8" max is all that is allowed. You may have voided the Lonestar warranty also.

It's easy enough to overextend the piston when in a bench vise. It's not easy once the master is installed. The brake pedal will strike the carpet before bottoming out the master.
If the master leaks now, it will only get worse when pressure builds up in the brake-lines.

 

I would first try, clean the back of the master with brake cleaner. Get it super clean and dry. Extra dry.
pump the brakes. Look. Hold brake pedal down with pressure and hold for a good minute, maybe 2. Check.
New fluid coming out?

 

For future use, I always found it easier to install the rear line FIRST, then push the master into the studs and tighten everything down. Then I connect the front line.

sorry to hear your woes. I also have over $30k in my c3, but it’s by far my favorite vehicle.

If you press the brake pedal can you hear a hiss and or does the leak get worse?

 

Yeah, I kind of learned the hard way. I did the front line first. I also found out that moving the wiper fluid bottle neck gave me a little room. I still don't know why the design didn't simply bring the lines in from the other side? I don't hear any hissing and it is a very small amount that appears to leak. Since I noticed it and dried it, then drove it and checked, each time it isn't really a stream, but when I run a paper towel up into the depression at the contact point of the MC and Booster, it comes back wet.

 

Yep, much easier and much less risk of cross-threading. I do the same thing with carburetors with the fuel line – get the nut started on the threads then mount it on the studs.

OP, this is a power brake car, yes? (Was manual even offered by '81?) You could have old fluid inside the booster diaphragm area (from the old master and a leaking rear seal). If so, I'm not versed in how to get it out but maybe someone else knows.

 

Ok, sounds like the master's rear seal is gone.
I would not use it until a better plan comes to mind. You Do Not Want the booster taking on Dot3.

There are kits to repair the master, new seals, piston, etc. Because its new, the condition of the bore is not an issue in your case.

Also, you really should have a gasket between master and booster. More of a dust shield than a seal but will keep dirt out of the booster.
Some Vette vendors will likely have it.

 

On my 1968 Corvette with power brakes I found it easier to make the brake line connections before attaching the master cylinder on the firewall. Some of the lines are more difficult than others but on the 1968 it was not a hard thing to replace the master cylinder. Getting the lines "right" that feed the proportioning valve was another story. The previous owner was not aware either as he ran the lines "however" he could get them to hook up. They look poor but function so I have left them alone for now. If it ain't Broke, Don't fix it.

I use a Phoenix Reverse Bleeder to "bench bleed" the master cylinders before installing the parts into the car. The Phoenix reverse bleeder makes bench bleeding "very easy" without risking damaging the seals. I do like to lubricate the seals with brake fluid before doing any bleeding.

The brakes system seemed like a major PIA when I first bought the Corvette C3. After decades of playing with the brakes I finally have a system that works well enough to trust it with aggressive driving. I have tried several tools that are supposed to "bleed the brakes" in some way, the goal being a hard brake pedal. The only one I recommend is the Phoenix Reverse Bleeder System. This reverse pump will get the air out more effectively than any other bleeding system I have tried.

Performing regular maintenance on the brake system like flushing the fluid every 3-5 years is crucial to a good working brake system. With the Phoenix you can do the whole job by yourself and in very little time. The only thing that you need to be careful with is the old brake fluid that is being pushed up into the master cylinder that you don't have fluid spilling over onto the painted parts in the engine compartment. I put a small plastic bowl that catches the excess and some towels around it to help clean any spills that might happen. Then using a Turkey baster I remove the excess fluid as I continue to push the new fluid up towards the master cylinder.

When you bleed your brakes, tap (gently) on the sides of the brake caliper as you first push fluid up from the particular caliper. "Gently" tapping can help release any air bubbles trapped inside the caliper body. I also suggest that people gently tap on the brass junction blocks used in the brake system. I have found more trapped air in the calipers after a Complete rebuild including separating the halves of the caliper than just a normal flushing of the fluid.

The Phoenix reverse bleeder is basically a hand pump that pulls brake fluid from the reservoir and then pushes the fluid into the brake system up to the master cylinder. You keep pumping new brake fluid until it arrives at the master cylinder. Any air trapped inside the brake system will go up with the fluid on it's way to the master cylinder where it is released. No more pushing air bubbles DOWN...