5ktq Brakes binding, MC? (long)

[Bernard Littau]

> Sorry about the length of this thing, but I thought I should get the
> details in there at the beginning.
>
> My 87 5ktq brakes are coming on while driving in hot weather, and they are
> binding enough to stop the rolling car when it is doing 3-6 mph. It is
[snip]

Hi Doyt,

The plunger is shown on 47.2 in the Bentley.  Figure 1 on 47.3 shows the
adjustment length.  Only adjust the length if it is incorrect.

Check that the brake pedal travel is unhindered.

Check that it is not a mechanical problem with the parking brake.

You already checked that the MC reservoir is vented.

If all the above are OK, then your MC is likely the problem.  (Well, it's
likely your MC anyways, as a few people have pointed out.)

I've not had a MC fail in this way, mine always seem to get too loose rather
than too tight -- the old brake pedal sinks to the floor at a stop light
problem.

For the brakes to bind as you describe, assuming the other stuff is OK, then
the MC must not be returning to its fully retracted position.  When the MC
is not being actuated, the internal cylinder is all the way to the back, and
the slits are open such that the brake lines communicate with the brake
fluid reservoir.  If the MC cylinder does not retract fully, these slits
stay closed.  In order to have the brakes bind, you need a combination of
the slits being closed, and some sort of pressure build up in the now closed
hydraulic system.  Enough heat will cause pressure in such a closed system
to make the brakes bind.

Sounds like the Audi MCs are prone to becoming sticky due to heat
distortion.  Scary.

I would think that one could note excess play in the brake pedal if the MC
were not fully retracting -- it would take more brake pedal travel before
one sensed the onset of real resistance.

Best,

Bernard Littau
Woodinville, WA
'88 5ktq