"One way of determining the master cyl size is working out
the volumes required."
Gil, the decision is slightly more complicated than simply the
volumes required at the wheel calipers and wheel cylinders.
Also involved is a decision as to the stroke desired of the
brake pedal.
In my case I chose the 80 mm diameter front calipers and the
1 1/16" rear wheel cylinders.
I chose these numbers because I desired an equal increase
in piston surface area both on the fronts and the back (I sometimes
drive on slippery surfaces and don't want to overbrake the back).
If memory serves correct one translates into a 30% increase
in surface area while the other is about 28%
The desired end result, (with no change in master cylinder
diameter) is approx 28 - 30% more pressure on the pads
and shoes with the same pedal effort.
This increase in pressure comes because of the increased
mechanical advantage. The increased mechanical advantage
comes from the brake pedal travelling 30% further.
This combination leaves at least a couple of inches before
the pedal would touch the floor. This is sufficient room for the
extra pedal travel required if 1/2 the system (either front or
back) was to fail.
Given the lack of service to the hydraulics of the brakes in
the past 25 years I was going to have to change (or rebuild)
the master cylinder anyhow. The need to fix became more
serious given the extra stroke I asked the old master cylinder to
provide. It failed almost immediately after completion of the
other changes.
The new master cylinder from Cinnibar is working great in my
application. With the different needs of the disk brake conversions
I would expect different solutions to be required.
I not only wanted less pedal effort for everyday driving but I
wanted much more serious action should an emergency
cause me to push the whoa pedal HARD.
I have made other changes as well. I have increased the rear
shoe width to 2 1/2" (25% increase in surface area) and the
front pads to Performance Friction Carbon Metalics in the largest
surface area available (about a 5 or 10% increase in surface
area)
My coach weighs in just under 12,000 Lbs. Only a light to moderate
pedal pressure is required under normal braking.
Although I have not yet been forced to get serious about it, when
the pedal is spiked hard I hear tires screeching so it seems to
work for me.
Regards
Gary Zingle
1973 GMC 26 foot
the volumes required."
Gil, the decision is slightly more complicated than simply the
volumes required at the wheel calipers and wheel cylinders.
Also involved is a decision as to the stroke desired of the
brake pedal.
In my case I chose the 80 mm diameter front calipers and the
1 1/16" rear wheel cylinders.
I chose these numbers because I desired an equal increase
in piston surface area both on the fronts and the back (I sometimes
drive on slippery surfaces and don't want to overbrake the back).
If memory serves correct one translates into a 30% increase
in surface area while the other is about 28%
The desired end result, (with no change in master cylinder
diameter) is approx 28 - 30% more pressure on the pads
and shoes with the same pedal effort.
This increase in pressure comes because of the increased
mechanical advantage. The increased mechanical advantage
comes from the brake pedal travelling 30% further.
This combination leaves at least a couple of inches before
the pedal would touch the floor. This is sufficient room for the
extra pedal travel required if 1/2 the system (either front or
back) was to fail.
Given the lack of service to the hydraulics of the brakes in
the past 25 years I was going to have to change (or rebuild)
the master cylinder anyhow. The need to fix became more
serious given the extra stroke I asked the old master cylinder to
provide. It failed almost immediately after completion of the
other changes.
The new master cylinder from Cinnibar is working great in my
application. With the different needs of the disk brake conversions
I would expect different solutions to be required.
I not only wanted less pedal effort for everyday driving but I
wanted much more serious action should an emergency
cause me to push the whoa pedal HARD.
I have made other changes as well. I have increased the rear
shoe width to 2 1/2" (25% increase in surface area) and the
front pads to Performance Friction Carbon Metalics in the largest
surface area available (about a 5 or 10% increase in surface
area)
My coach weighs in just under 12,000 Lbs. Only a light to moderate
pedal pressure is required under normal braking.
Although I have not yet been forced to get serious about it, when
the pedal is spiked hard I hear tires screeching so it seems to
work for me.
Regards
Gary Zingle
1973 GMC 26 foot
