Disc Brake Conversion

Dan W. said:

1968 cougar, power drum front brakes. I bought a parts car with standard disc brakes.

Click to expand...

If the parts car is also a '68 Cougar, then yes, you can swap the disc brakes over. The best way to do that (as Jim said) is to swap the spindles and all. You should swap the proportioning valve from the disc car as well, because disc brakes are proportioned differently from drums. I am reading that the disc car is not boosted. There should be no problem there. Use the booster and master which are already on the drum car.

If you do not do a proportioning valve, the rears will lock up before the fronts. An adjustable valve makes it so you can tune them for your driving.

Your all drum brake car will have a pressure differential valve (just follow the lines from the MC and it will lead you to the valve). Other than distributing the brake fluid through it, for the front and rear brake circuits, it only has one function; to sense for a difference in brake pressure between the primary (front) and secondary (rear) brake circuits. As long as brake pressure is equal, the valve spool inside the valve body will remain centered. If there's an imbalance in pressure --such as a rear brake hose fails or wheel cylinder blows out, the imbalance in pressure will put more pressure on the end of the primary side of the valve spool that in turn will force the spool over to the lower pressure (failed) side. This will close the contacts on the valve switch which will complete a ground path through the pressure differential valve body and illuminate the brake warning light on the dash.

On the disc brake parts car, there will be a disc/drum combination valve. It will also have a pressure differential valve spool inside but, it will also have a proportioning valve to limit the pressure rise to the rear drum brakes in the event of a panic stop situation.

You can transfer the disc brake components and booster over onto the drum brake car. You will need to swap out the drum brake pressure differential valve with the disc/drum combination valve.

Depending on whether or not the drum/drum MC is the original or whether it's a parts store replacement, there may not be any residual pressure valves (RPVs) in the primary or secondary ports of the drum/drum MC. Most parts store replacement drum/drum or disc/drum MCs don't come with RPVs anymore even if the OEM application did.

However, I wouldn't recommend using the donor car MC or the current drum/drum MC. It would be best to buy a brand new (not rebuilt) disc/drum MC. A drum/drum MC may actuate the disc/drum circuits initially but they will not have enough fluid reserve for the disc brake circuit, as the disc brake pads wear down. As the pads wear down, the fluid reserve of the primary MC reservoir will drop. A disc/drum or disc/disc MC has larger reservoirs (more fluid capacity) than a drum brake MC to compensate for this. A drum/drum MC doesn't have the capacity to support the amount of fluid and fluid reserve needed for a disc brake system.

Unless you have a set of brake gauges, to adjust the pressure setting on a manually-adjustable proportioning valve, then you have no way of knowing exactly what it's being adjusted to. Adjusting the knob after a few hard stops will just be a guess at best.

If you do install a manually-adjustable proportioning valve, it can be installed with the drum/drum pressure differential valve. Don't use it with the factory disc/drum combination valve without eliminating the OEM proportioning function of the disc/drum combination valve. If you use the (unmodified) combination valve and a manual proportioning valve together, then you will have two proportioning valves competing against each other ....and that's no good.

what is the reason your converting to disc?

I'm confused about the need to change spindles: Are the Cougar's spindles different from the 67-70 Mustang spindles? It is my understanding that 67-70 Mustang spindles for drums and discs, 6 cylinder and 8 cylinder are exactly the same. My 6 cylinder 67 Mustang converted from 4 lug drum to 5 lug disc using a CSRP K-H conversion kit and it went perfectly using the same spindles.