Just air in the system or a bad master cylinder?

[jdbubb at verizon.net (Dan Bubb)]

I assume there are no apparent leaks anywhere.
I would think that with all the bleeding you've done that even if there was some air in the system you would still be able to pump up the pedal and get some sort of pedal height just by pumping the pedal very fast multiple times. If you can't even do that then your MC is probably toast.

You didn't say if the replacement MC was a new unit or not. Used brake components tend to build up crap and crud on the surfaces that are not swept by the pistons and seals. If you then make the piston travel further than it generally has (such as when bleeding the system and using full strokes) the seals are dragged across the crud and can be damaged.
So, manual bleeding with a used MC has a pretty high chance of damaging the MC seals unless the system that it came from was well maintained and had the fluid replaced regularly.
Unfortunately I've also damaged a brand new MC by manual bleeding.
I personally don't think priming or bench bleeding the MC is necessary.
I do highly recommend using a power bleeder! They aren't that expensive, avoid potential damage to even a new MC from manual bleeding and turn bleeding A3 calipers that need to turned upside down to bleed into a one man job!!
HTH
Dan

From: "brad boston" <boston813roc@gmail.com>
Sent: Thursday, November 16, 2006 12:39 PM
Subject: Just air in the system or a bad master cylinder?


> The saga continues...
> 
> thus far in our story the shade tree mechanic (me) and his trusty side kick
> (a friends that brings cold beer) have had no luck in getting the brake
> system to work on my (now garage queen) '88 16v.
> 
> the Things that have changed or had done to it are as follows
> 
> Replaced the rear calipers with some sweet looking red powder coated ones
> (thanks Peter)
>  This got the E-Brake working and was the initial cause of this adventure
> 
> Replaced the front calipers with nice red ones so everything would match
> (they look great on the car)
>  Tried to bleed down the system but the pedal went right to the floor after
> bleeding all four calipers :(
> 
> Though it was the master cylinder so I ordered a new one and some stainless
> Steel lines just because I already had the system apart.
> 
> Tried to rebleed the system but again no luck
> 
> Asked for help and found that the master cylinder needed to be primed before
> it is installed
>  used a short piece of brake line and some tubing to bleed each outlet of
> the master cylinder (This got a lot of air out)... But still the pedal goes
> right to the floor
> 
> Tried to rebleed the system at all the calipers but still nothing
> 
> I am starting to think that I may not have gotten a good master cylinder or
> that there is some air in the system that I was not able to get out even
> with repeated bleedings
> 
> OK, so what do you all think?  Should I try rebleeding the master cylinder?
> Try replacing the master cylinder with a known good one? (any one have one I
> can borrow?)  Any other ideas or suggestions would be appreciated.
> 
> Tune in next week to find out if the shade tree mechanic will be ready to
> take the scirocco out for the winter season of auto x.....
> 
> Brad
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