Ray's Chevy Restoration Site

Nova Disc Brake Swap Notes

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

Master Cylinder

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

Pressure Differential Switch
This switch operates the "brake" light on the dash in the event that one half (front or rear) of the braking system fails due to pressure loss.
Distribution Block
This consists of a pressure differential switch and a "T" fitting for the two front brake lines.
Residual Pressure Valve
This valve is used with drum brakes. It holds about 10 psi of line pressure in order to keep the shoes close to the drums. These valves are located in the master cylinder outlet ports or in the rear (drum brake) portion of the combination valve.
Metering Valve
This valve is used to slightly delay the application of the front disc brakes to help reduce pad wear and help rear end stability.
Proportioning Valve
This valve is used to prevent rear wheel lock-up due to weight shift during braking.
Combination Valve
This combines a proportioning valve, metering valve, residual pressure valve, and pressure differential switch in one unit. It also serves as a distribution block.

Master Cylinder, Valve, and Switch Usage

1966 & earlier drum/drum
These systems use a single chamber master cylinder. The lines are "T'ed" off to each wheel without the use of any valves or switches (other than the residual pressure valve in the master cylinder).
1967 - 1974 drum/drum
These systems use a dual chamber drum/drum master cylinder. A distribution block serves as a "T" for the front lines and contains a pressure differential switch.
1967 - 1970 front disc/rear drum
These systems use a dual chamber disc/drum master cylinder. They also use the same distribution block as the 67 - 74 drum systems. However, they have a separate metering valve added in the front brake line. They did not use a proportioning valve.
1971 - 1979 front disc/rear drum
These systems use a dual chamber disc/drum master cylinder. A combination valve takes the place of the separate distribution block/switch and metering valve. The combination valve also incorporates a proportioning valve that is not present on earlier systems.