On Fri, 31 Oct 2003 07:36:18 -0700, you wrote:
>Subject: Re: [oletrucks] Questions on Mods to 52 Burb
>Operating disk brakes requires much more fluid movement. That's why the
>front reservoir is always so much larger on disk/drum cars. I think you
>need a disk/disk type master.
Ummm...no. The front reservoir is larger because the calipers
self-adjust by displacement of the piston towards the rotor as the
pads wear, thus increasing the volume behind the caliper piston. The
intent is to have enough fluid in the reservoir to last the life of
the pads even if the operator never checks the fluid. Drum brake
adjustment is mechanical, at the shoes, and (provided the adjustment
occurs properly) the wheel cylinder pistons do not change location as
the shoes wear, so no additional fluid is needed for them over time.
The amount of fluid that moves when the brakes are applied is
determined by the diameter of the master cylinder bore. In almost all
cases, the caliper and wheel cylinder diameters are selected by the
engineers so that they can work with the same diameter master
cylinder. Straight-bore masters are cheaper to build than step-bores.
The big issue here will be whether the system has appropriate pedal
travel. If the pedal goes nearly to the floor, a larger diameter
master cylinder should be used. If it is a high, hard pedal, a smaller
one should be used. My guess is that the OP is probably OK with the
parts he has chosen.
Another issue is pedal effort. Often a given (older) model of car
would have a 1" master when equipped with manual brakes and a 1-1/8"
master when equipped with power brakes. The smaller manual cylinder
will give greater braking effort for the same pedal pressure at the
cost of increased pedal travel. The larger power master will require
greater pedal pressure, but that's OK since the booster will help push
the pedal. At the same time, the pedal travel decreases.
It's all a balancing act. If you're starting from scratch to convert a
drum vehicle to discs, it's best to select parts from a late model
vehicle that is about the same weight and has about the same weight
distribution as the one you're upgrading. Then the factory engineers
have done all the work for you. If you mix and match parts, you just
have to try it out and see how the pedal feels and how well the brakes
Provided one is willing to assume responsibility for checking brake
fluid once in a while, any configuration of master (disc/drum,
drum/drum, or disc/disc) will work as well as another. The difference
from one to another is all about volume of the reservoir. Except....
Some of the older dual circuit masters, from the late '60s mostly,
have residual valves installed under the tube seats. Early disc/drum
masters may have only one valve, in the rear brake port of the
cylinder. If you use one end (or both ends) of one of these with
discs, you need to remove the residual valve(s). Use a bit of wire
such as a straightened paper clip inserted gently into the port to
check for valves. If one is present, the wire will go into the port
only a quarter-inch or so, and will stop with a rubbery feel. If there
is no valve, the wire will go in an inch or so.
Heather & Joe Way
Sierra Specialty Automotive
Brake cylinders sleeved with brass
Gus Wilson Stories
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959