1973 IH Scout ll drum to disc front and back kit?

[Mike T]

Moses,

My brother Stephen is in New Mexico and is redoing a 1973 IH Scout ll. He is looking for a total kit to change out the drum brakes. He's trying to find front and rear kits from maybe the same manufacturer that may have everything including the master, brake lines, and everything else nessessary. Thanks for your time and consideration. Mike T

[Moses Ludel]

Mike T...There is real merit to doing the front disc brake conversion, especially with an open knuckle front axle.  As for the rear, unless the vehicle is fording streams or continuously on dusty dirt roads, the gains in brake performance are negligible.  Most trucks until the late nineties and early 2000s were still disc front with drum rear brakes.  Brake bias is toward the front, so drum rear brakes are plenty adequate.  However, a 1973 Scout II rear drum/shoe size can be 11" x 1.75", which is not great.  11" x 2" or wider shoes would be desirable.  Factory parts (backing plates, wheel cylinders, drums, shoes, etc.) from a later Scout II (1975-up) could upgrade the rear drum brakes as long as the axle shaft flange stick-out is the same.

So back to your brother's desire:  four-wheel disc brakes.  I found similar parts at Scout Parts (https://www.scoutparts.com) and D and C Extreme.  Here is the link to a front brake conversion:  https://scoutparts.com/Disc_Brake_Conversion_Kit_Front_4x4_4x2_Scout_II_Dana_30_ONLY__Scout_II_Scout_II_Diesel/p17367.  Read the footnotes!  This is for a Dana 30 front axle and requires specific wheel rims to clear the brake calipers.

For the rear disc conversion, E-brakes and a proportioning valve enter the discussion.  Here is D and C Extreme's kit that takes that into consideration:  https://dandcextreme.com/product/scout-ii-rear-disc-brake-kit/.  Scoutparts.com offers a similar kit with options:  https://scoutparts.com/Rear_Disc_Brake_Conversion_Kit_Rear_Scout_II__Scout_II_Scout_II_Diesel/p17360.  Similarly, D and C Extreme offers a front disc conversion kit.

Note that the proportioning valves both offer are actually manual metering valves.  With experimentation and adjustment, they do work, although this is not the same as an OEM combination valve that proportions and applies rear brakes slightly ahead of the front in timing.  Both act as proportioning, but OEM brake system valves have more functions than a manual, adjustable proportioning valve.  Think of a manual, adjustable proportioning valve as reducing or increasing the apply pressure to the rear brakes.  Once adjusted as desired (or the best compromise), that pressure is fixed.

Another way to do this upgrade, especially with disc front/drum rear, would be OEM or stock Scout II parts from a later model.  You would find a donor vehicle or a parts catalog with OEM replacement parts and update every piece in the braking system from the master cylinder and combination/proportioning valve to the caliper brackets, calipers, rotors, pads, etc.  This requires careful review of all parts to make sure they match up with the front axle, rear axle shafts and so forth.  Use of wider rear drum brakes may require finding the right backing plates, wheel cylinders, shoes, hardware and drums, making sure these parts will work with the 1973 axle shafts and their flange stick-out length.

The disc conversion kits seem to be a one-stop shopping venture.  However, any four-wheel disc brake conversion is a "project".  There will be a need for detail and attention paid to brake hose lengths and use of the correct master cylinder.  As a point here, a disc/drum brake master cylinder may have a residual check valve on the rear brake port to maintain slight pressure to the rear brakes even when the pedal retracts.  The disc front brake circuit does not have a residual valve, as residual pressure would cause disc brake pad drag on the rotors. 

Rear or front drum brakes have heavy shoe retraction springs that overcome the residual pressure and allow shoes to retract completely while still maintaining slight pressure in the brake lines.  The residual pressure is essential for keeping drum brake wheel cylinder cups expanded, preventing fluid seepage and keeping air out of the cylinders.  There is also no lag (other than shoe-to-drum clearance) when the brake pedal is applied.

The current master cylinder in the 1973 Scout II is designed for four-wheel drum brakes and uses residual check valves.  Note the #10 "Reservoir Valve Spring" at the drum brake master cylinder.  Outboard of the spring is a rubber valve (residual pressure valve) that gets pressed against the port seat to hold a small amount of pressure in the lines of a drum brake system.  The disc brake master cylinder (unfortunately not 1971-up as indicated) would not have a residual valve on the disc front caliper circuit.  This disc/drum system would, however, need a residual valve on the rear drum brake system with wheel cylinders:

On disc brake conversions, if the master cylinder is from a drum brake application (like the 1973 Scout II), the residual check valve(s) would need to be removed from the port to the disc brakes.  If the conversion is to disc front brakes only, the check valve would remain in place at the rear drum brake port.  The port seats are removable, often done during a master cylinder rebuild.  A thorough rebuild kit often contains new port seats and instructions on how to remove the old brass port seats.

When choosing a master cylinder for a disc brake conversion, be certain that the fluid reservoirs, bore and piston diameter(s) are sufficient for disc brakes.  A disc brake master cylinder is likely to have a large reservoir(s) and larger piston and bore displacement.

Moses