<VV> Dual or Single Master Cylinder

[JVHRoberts at aol.com]

Yep, I am aware of both the stepped bore design, as well as dual cylinders  
with a balance beam. 
My comment was, you leave only one bleeder open on a Fiero, and the pedal  
goes damn near to the floor! And this is on a 1988, with the VASTLY improved  
brakes! 
On three of my four cars, the modus operandi with one bleeder open is NOT a  
pedal going to the floor. There's plenty left on the remaining circuit. I 
think  I understand how Nissan does it, but the Subarus will require more study... 
Anyhow, I have a four disk setup I plan on installing sitting on the  shelves 
in my garage and tool shed. I already have them mounted on LM rear  trailing 
arms, complete with internal drum parking brakes and four piston  calipers! I 
am working on the four piston setup for the fronts... the rotors and  hubs are 
already machined together, now it's a matter of getting the caliper  bracket 
right... 
 
 
In a message dated 12/4/2005 11:03:17 P.M. Eastern Standard Time,  
lechevrier at earthlink.net writes:

>  > Yes, I know [the Fiero] has a seperate plastic reservoir, but I don't  
think the guts work materially differently from a Corvair dual MC <  <

John --

There is a difference! Maybe not in the basic idea  that they are all 
hydraulic brake cylinders, but the Fiero, like many other  GM's of the era has a bit 
of a twist on the design. I cannot at the moment  remember their 'name' for 
this feature, but GM redesigned their disc brakes  for a lower rolling 
resistance or drag by giving the piston seal a bit of an  angle, causing the piston to 
retract further providing more clearance between  the pads and rotor.

This 'feature' required more hydraulic fluid volume  to activate the brakes, 
so GM designed their 'stepped master cylinder' with an  internal compensating 
valve. There is a website that goes into great detail  about this system, some 
of the problems it causes, and some of the cures. This  site is bookmarked on 
a different hard drive that is not in use, so I am sorry  I can't reference 
it. Anyway, the Fiero, with four wheel discs brakes, all of  which are of this 
design, barely has enough pedal travel to adequately service  the brakes when 
all is in good repair, and is barely marginal (which is a  whole lot better 
than nothing) with a partial failure.

You want to be  VERY familiar with all of this if you want to adapt one of 
the stepped master  cylinders and its pre-pressurizing feature to any use other 
than what was  intended, but for the low drag brakes found on many 80's GM 
vehicles, they are  the oem design.

I believe that the owners manual for most dual baking  system cars (Volvo 
does, or used to do, it a bit differently) does say that it  offers reduced 
braking performance under conditions of partial brake failure.  So, "Blow a 
circuit, and you really don't have much left," is the normal modus  operandi. I 
sincerely hope that I never again have the personal experience  from which I speak.

Perhaps one would want to consider a true dual  master cylinder setup, ala:
http://www.stoptech.com/tech_info/PedalSetup-DualMaster-Guide.pdf,
also  perhaps considering professional installation.

Drive On!

Bill  Strickland