200tq Braking Problem

[Kneale Brownson]

At 11:25 AM 05/22/2003 -0700, Darek Jarski wrote:
>Among other issues, my '89 200tq developed a braking problem. I am getting
>deep pulsating pedal feel and loss of braking power [intermittingly but
>progressively worse]. The pedal feels a bit "spongy." There is also some
>free play in the brake pedal. I have a GOO leak in the rack [next on my long
>list] and possibly a bad bomb, however I thought that loss of hydraulic
>pressure would lead to a stiff pedal [correct?]. I am not losing any brake
>fluid. Could my master cylinder be bad? How do I test it? What else should I
>look at?
>
>I am not at all familiar with how the braking system interacts with the
>hydraulic system, so if anyone knows of a good write-up or is willing to
>explain it I would greatly appreciate it.

You have a hydraulic brake system where a piston in the master cylinder
pushes on fluid that then pushes on calipers at each wheel to clamp the
pads onto the rotors.  Behind the master cylinder is a brake booster that
amplifies how much pressure your efforts on the brake pedal supply to the
piston in the MC.  That brake booster gets its power from the hydraulic
pump run off a belt on the front of the engine.  To assure that you have
boost even in the event of the engine failing while you're moving, there is
a brake pressure accumulator that retains a supply of pressurized oil to be
utilized in operating the booster.  If the brake pressure accumulator
(fondly referred to as the "bomb" because of both its shape and it's
propensity to disgorge its contents explosively if not discharged before
being disconnected from its lines) is not functioning properly, the minute
the engine stops, you have only the basic hydraulic brake system with no
boost.

So let's say you have a bad bomb and you push in your clutch so your engine
slows while you apply the brakes.  With insufficient RPM's, the hydraulic
pump cannot supply the pressure level required to make the booster function
as it should, so I'm guessing you may feel the boost varying in the way the
pedal feels.  You can test the bomb by turning off the engine and counting
the number of pumps on the brake pedal before you feel it stiffen.  A good
bomb returns 30 or so pumps.  A poor one maybe 10.  A dead one may give you
no pumps.

All that said, your description of a mushy pedal sounds like a failing
master cylinder.  If, with the engine off, you push on the pedal and it
keeps wanting to move toward the floor, that's an indication of failed
seals in the MC.