Rick Yeah All that technical stuff is a little confusing. What I get
from it is; Check all brake surface and replace,Replace master cyl so I wont
have to worry about that being a problem and make sure to get all air out of new
HD brake fluid. As for the spindle rethreading die, none of the parts houses or
tool supply houses knew anything about it. the spindle is 27/32 20 thread and
since the threads are pretty well scuffed off I should be able to rethread w/
13/16. Does that sound logical?
Bill
>
> > I asked a brake question last week and got no reply How come
> > my front brakes dont work when I lose rear brakes?
>
> Bill,
> Think I touched on this a few months back, but briefly: A so-called
> "dual" master cylinder (like ours) has one piston attached to the pedal, and
> another floating piston further down the same bore. As you depress the
> pedal, the primary piston passes the compensating port (little hole
> connecting to the reservoir) and starts to build up pressure. Fluid is lead
> from this chamber of the cylinder via a pipe to one set of brakes. Pressure
> in this primary chamber also presses against the secondary (floating) piston,
> and IT moves down the bore, passing ITS compensating port and building
> pressure in the secondary chamber, where another pipe leads to the OTHER set
> of brakes.
>
> If either set of brakes (front or rear in our case) springs a leak, the
> corresponding chamber empties, and that piston "bottoms out". If it is the
> primary piston, it moves down until it physically touches the secondary
> piston, and now pressure is applied directly to the secondary piston. If it
> is the secondary chamber that empties, the secondary piston bottoms at the
> end of the cylinder bore, and pressure builds up between the two pistons in
> the primary chamber. In either case, THEORETICALLY you still have the
> remaining half of the brakes. (In fact, the DOT required manufacturers to
> install a brake warning light to remind the poor befuddled driver that a
> failure had occurred, since this system worked so well we might not notice
> the problem otherwise!)
>
> HOWEVER, in the real world it doesn't work that well. Usually the problem
> is brakes which are out of adjustment, have a little air trapped in lines or
> cylinders, or spongy hoses, etc. Any/all of these things cause a low pedal.
> When one of the sets of brakes fails, you lose about 2/3 of the pedal travel
> before its piston "bottoms out". If the remaining brakes are a bit
> low/spongy, then the pedal hits the floor before the remaining brakes fully
> apply. (Your case.) Sometimes pumping the pedal will bring it up enough to
> work, but the GMC is hard to pump effectively. Also, the GMC brake system
> works hard even if it's all there, so any failure is more dramatic than in
> your car.
>
> BTW, the warning light/proportioning valve has little or nothing to do
> with all this, except as another place to trap air bubbles, or as as a
> potential leak between front and rear systems which might circumvent the
> intended separation.
>
> Hope this is not too confusing, and helps a little.
>
> Rick Staples
> '75 Eleganza
> Louisville, CO
