This page contains my notes on the front drum to disc brake swap on my '74 Nova. These notes are intended to supplement the info found on LeadFoot's Nova Disc Brake Swap Page.
Notes on Fitting Sizes
As is mentioned on
LeadFoot's Nova Disc Brake Swap Page, the rear brake line uses a different
size flare nut fitting for drum and disc applications. He mentions cutting off
the old fitting, installing the one salvaged from the donor car, and re-flaring
the line. An alternative is to use an inverted flare adapter. I found that a
Weatherhead fitting #7908 will adapt the
1/4" rear brake line to the 9/16"-18 thread used in the combination valve.
The left front brake line on drum brake cars uses an oversized flare nut on the distribution block end. I dealt with this by replacing the front steel brake lines with a new pre-bent set. That also required changing the subframe brackets (at the hard/flex line junctions). Drum brake cars have these brackets located on the sides of the subframe while disc brake cars have them on the top edge of the subframe.
Brake System Components
Manual and power brake master cylinders typically have different bore/piston
sizes. Manual brake master cylinders used on Novas usually have a bore/piston
that is either 7/8" or 1" diameter. This small diameter makes the pedal
relatively easy to push. Power brake master cylinders used on Novas have a
larger bore/piston (about 1-1/8" diameter) that pushes a greater volume of
fluid with less pedal travel. It does increase the pedal effort but that is
more than compensated for by the booster.
Another difference between manual and power brake master cylinders is the depth of the push rod hole in the back of the piston. Most manual brake master cylinders have a deep hole to retain the push rod. There are exceptions. 1975 - '79 Novas with manual brakes used a master cylinder with a shallow hole and a special rod and retainer to hold it in place.
Drum and Disc master cylinders differ in the presence or absence of a residual pressure valve. All drum/drum brake master cylinders have built in residual pressure valves to hold about 10psi in each (front/rear) line. Some disc/drum master cylinders have a residual pressure valve in the rear drum brake outlet. These were used on the '67 - '70 Nova factory disc brake setups. Other disc/drum master cylinders do not have a residual pressure valve since they are used in conjunction with a combination valve containing a residual pressure valve. These were used on '71 & up Nova factory disc brake setups.
Most disc brake master cylinders also have a larger reservoir for the disc brake circuits since the disc calipers require a larger volume of fluid than the drum brake wheel cylinders.
Valves and Switches
Master Cylinder, Valve, and Switch Usage