-=> Quoting Day Brown to all <=-
 DB> I had always seen the dual reservoir on master brake
 DB> cylindars as one of the handful of good ideas in all
 DB> As I understand it, the idea was for the brake pedal
 DB> to push on two pistons packed in the same bore, one
 DB> for the front brakes the rear for the rear.
Actually, a single piston that has two sets of seals which moves fluid
through two lines to the proportioning valve.
 DB> Now, I got this redneck wreck, an 85 olds 98.
 DB>  and it felt just like a lost a brake line on
 DB> an old pre-dual reservoir brake system.
It should have two reservoirs in the single unit.  GM, as others US makes,
have been like this for decades.  The early ones were in a single casting
which later changed to a plastic dual reservoir.  Some makes may have
two separate plastic reservoirs that are functionally the same.
 DB> The master cylindar was not full, but neither was it 
 DB> so low as to trigger the fluid level sensor.
I doubt it has a level sensor, but possible.  It does have a switch in
the proportioning valve, that triggers if either front or rear system
loses pressure.
 DB> Anyone know what new idea some twit at Olds had that
 DB> would screw up a failsafe system? Judging by the grime
 DB> on the aluminum casting, it looks like an OEM part.
Open it and count the reservoirs.  I think you'll find two, same as any
other car of the era.  If it failed completely, you probably had severe
rust/corrosion in the M/C bore or a line failure.  When replacing it,
flush the entire system to remove any moisture and replace the fluid
with new fluid from unopened containers.
--- Blue Wave/386 v2.20 [NR]
---------------