[T3] I still need to change the brake fluid.

[Jim Adney]

On 30 Jul 2018 at 20:28, William Jahn wrote:

>  I've read on old master cylinders like mine certainly is that if one
> bleeds brakes moving the pedal until it's is down all the way to get
> the air out if the master bore has any pitting it can ruin the seals. 

Rust pits in MCs occur in 2 places: right at the entrance to the bore, and 
under the inlets from the reservoir. These are places that see higher 
concentrations of water in the brake fluid. The former is a place the first seal 
must cross every time it moves even a little bit. The latter is a place the seals 
will never reach.

Replacing the fluid in the reservoir is a very good thing to do, because that's 
the fluid that accumulates the most moisture. Slowly draining the rest, as you 
keep the reservoir full, is an easy way to make sure that most of the fluid in 
there is dry.

You can pump if you like; it won't do any more harm than normal driving, as 
long as we're talking about a car that gets driven occasionally. (Something 
that hasn't been driven for a decade is a different matter.) I recommend 
pumping because it is more likely to stir up dirt in the system and flush it out.

Draining from the bottom bleeders on your '73 allows you to get rid of most 
of the old fluid in your calipers. That won't happened with a simple flush.

The caliper with a torn boot should be completely rebuilt. The tear lets dirt, 
water and humidity get into the space between the boot and the seal. That's 
where rust will form and seize the piston. I don't advise just replacing the 
boot. It doesn't need to be pretty; it just needs to be clean and the rusted 
bore cleaned up  

In general, I recommend rebuilding both calipers, then refilling with DoT-5 
Silicone Brake Fluid. The very best way to do this is to make some caps for 
the front brake hoses, so you can cap them off and open each one in turn 
and flush and bleed the system. Once that's done, attach the calipers and fill 
them with clean fluid. This avoids flushing dirty old fluid thru your nice clean, 
freshly rebuilt calipers. I make caps for this purpose from the female ends of 
dead brake hoses.

No need to replace hoses unless they are bad. Good hoses will allow a 
steady stream of fluid out of the bleeder. Bad hoses won't allow fluid out until 
you pump the pedal.

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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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