Brakes - 5K wagon

[Bernard Littau]

> > A bad bomb won't cause the pedal to sink to the floor under
> > use.  A bad
> > master cylinder WILL.
> >
>
> Brand spankin' new master.  To help isolate this even further,
> here is what
> the system has now:
>
> - new calipers & pads
> - new flex hoses all the way around
> - new rear wheel cylinders & shoes
> - new e-brake cables
> - new master cylinder
> - new brake fluid, all the rest was drained
>
> Current behaviour:
>
> start the car, no brake alarm, press brake solid.  Can move the car with
> brake, still solid.  Now stand full force on the brake, it will
> slowly fade
> to the floor and then the brake alarm comes on.
>
> If anyone needs me to do something extra to help isolate, speak up
> please.......

Hi Dave,

Isolate which signal is causing the brake alarm to fire:
1) Brake Fluid level
2) Hydraulic Fluid level
3) Low pressure at the Brake Booster

You can disconnect two of these at a time and do your reproduce case to see
when the alarm occurs.

Also, just for fun, how many pedal pumps does your bomb hold?  Run the car
for a minute or so to build up hydraulic pressure in the bomb, and then shut
off the car.  With the car off, pump the brake pedal until it gets hard,
counting pumps.  What do you count?

I am thinking you have a bad bomb or weak hydraulic pump, and that the brake
alarm signal is being sent by the low pressure at the brake booster.

I cannot see how a bad a) Bomb, b) Hydraulic Pump or c) Brake Booster could,
individually or in conjunction with each other, be the primary cause behind
a brake pedal sinking to the floor.

If the brake pedal is sinking to the floor it is either a bad Master
Cylinder, or some problem downstream from the MC (air or a slave leak, etc).

The power booster does allow you to exert more pressure on the brake system,
and sustain that pressure longer, than you can without a power booster.  It
is possible that if you pressed hard enough and long enough with the car
off, your brake pedal would also sink to the floor.

A new MC does not rule out a bad MC; it does make it less likely, however
:-)

Best,

Bernard Littau
Woodinville, WA
'88 5ktq