[T3] Brake failure ‘67 Fastback

[Jim Adney]

On 11 Feb 2018 at 22:08, Jesse Hernandez wrote:

> I lost brakes and brake fluid was coming out of the reservoir.  Pedal went all
> the way to the floor.  After a short time I was able to get some brake pressure.
>  My mechanic was unable to duplicate the brake failure and was unable to
> identify the cause of the problem.  He replaced the master cylinder and the
> brakes worked fine.  This past weekend the brakes failed again while I was on
> the freeway.  Fortunately I was able to reach the shoulder and use the handbrake
> to stop.  The brake fluid was again coming out of the reservoir.  After a short
> time I was able to get some brake pressure.  What should I be looking for as the
> cause of the brake failure?

That's about as typical (and extreme) a brake failure as anyone ever 
sees. At least one wheel brake is dragging all the time. This 
overheated enough to boil the brake fluid in it. When the fluid 
boiled, that left part of the system filled with vapor that was 
easily compressed (so it was impossible to develop any pressure in 
the system) making the pedal ineffective. When the fluid boiled, it 
pushed the fluid above it out of the top of the reservoir.

Once things had time to cool down, the vapor condensed and you got 
your brakes back.

The solution is to figure out which wheel (or wheels) is/are over 
heating. It's not the fault of the master cylinder. You should be 
able to drive the car a short distance with minimal intensional use 
of the brakes and then check all 4 wheels to see which one is extra 
hot; reach thru the slots in the wheels and touch the disk or drum. 
Be careful, don't burn your finger; the bad disk/drum will be 
extremely hot.

It IS possible that someone has misadjusted your brake pedal (or rust 
is preventing it from returning all the way.) There should be a bit 
of free play in the brake pedal when you move it by hand: It's 
usually about 1/2" at the pedal, but the important thing is that 
there needs to be SOME free play there.

If it's a front wheel, you either have a sticky piston in a caliper 
that needs to be rebuilt or a hose that's collapsed inside and needs 
to be replaced. If it's a rear, it could be a hose, or it could just 
be that the shoes have been adjusted too tight, or the wheel cylinder 
is sticking, and should be rebuilt.

This is a simple problem which you have described very clearly. Your 
mechanic should have figured this out. You need a better mechanic.  

I rebuild calipers. If the problem is in the rear cylinder, the 
easiest thing to do is to rebuild it right on the car. It's easy.  

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