This is one of a series of notes about refurbishing a 1950 Cheverolet series 3100 pickup truck for may son, who intends to use it for daily transporation. The OEM brakes seemed to be in reasonably good shape, but a careful inspection was surely in order.
The parking brake system looked like a good place to begin. Sure enough, the levers on the pivot arm (the heavy bar that runs transversely across the truck chassis) had been improperly welded by the factory. Amazing the things they got away with in those days. Two iron (!) bearings supported the pivot arm on the chassis. Replacement bearings were fabricated from 3/8" aluminum plate, which restored clearances and dampened a major rattle. The foot-pedal assembly was taken apart, cleaned and painted. The mechanism works now: press down the pedal and the brake holds at any position selected; pull the release knob and the brake pops off.
Pedal Assembly
While not strictly a safety modification, the clutch and brake pedals wobbled annoyingly on their common shaft. The pedal assembly mounts on a heavy iron pressing, which can theoretically be unbolted from the frame with the cab in place. Five hours and several bruised knuckles later, I got it off.
Pedals ride on thin-wall brass bushings, sized to match the 1" OD shaft. Replacement bushings are available by mail, but won�t help much if the shaft is also worn, as it was in this case. There appears to be no aftermarket source for pedal shafts. Rather than fabricate a new shaft and wait for the mail order bushings, I purchased two 1 1/4" OD x 15/16" ID brass bushings locally and machined them to fit. Pedal bosses were opened up to allow a thicker bushing wall and bushing ID was reamed to match the worn shaft diameter. If you have this work done, impress on the machinist that bushing-to-shaft clearances should be tight, on the order of .001." More slop than that results in a noticeable wobble.
New Grade 8 fasteners, daubed with assembly lube, should make removing the pedal bracket easier for the next poor devil to try it.
Brake Shoes
An earlier wheel bearing failure had pounded the lining on one front brake shoe into pulp. It looked like cardboard, and that set me to thinking. What is the tensile strength of brake lining anyway? And what about the other criteria such as durability, coefficient of friction, noise generation and so on? A visit to the local NAPA parts store failed to produce so much as a listing for 1950 Chevrolet brake shoes. Ditto for Big A. Brake Check said they would be happy to reline the front brakes, but when I asked about nonasbestos linings, the service writer demanded to know why I was interested in that product. "So I�m a tree-hugger." He said he would call back, but never did.
As it turns out, aftermarket brake components are exempt from federal regulation. Maybe those brakes were made out of cardboard. Neither are manufacturers required to provide consumers with information about the composition, performance or durability of their products. US-made linings may include some little information in the form of an SAE alphanumeric code printed on the edge of the linings and visible when assembled to a brake shoe. Reading from left to right, you will see the name of the manufacturer followed by series of numbers that relate to the composition of the lining. Unfortunately the significance of those numbers is secret. The last two letters translate the cold and hot coefficients of friction of the linings. The higher the Cf, i.e., the closer the number approaches 1.0, the greater the friction between the lining and drum.
Code Cofficient of Friction (Cf)
- C 0.00 - 0.15
- D 0.15 - 0.25
- E 0.25 - 0.35
- F 0.35 - 0.45
- G 0.45 - 0.55
- H 0.55 and above
- Z ungraded
A brake lining marked HF generates a Cf of at least .055 when cold. Hot performance drops off to something between 0.25 and 0.35. All things equal, the higher the Cf, the less pedal force required to stop the vehicle. High coefficients of friction used to mean rapid lining wear, but that is not necessarily true of modern materials. However without the wear data, one cannot say.
Your only assurance of quality is to purchase the best name-brand lining available. The catch is that top quality linings seem to be reserved for modern high-performance cars. I cannot prove it, but I believe that all relined brake shoes sold for vintage vehicles contain between 30% and 50% asbestos This appears true of brake shoes ordered by mail, sold over the counter or installed by specialty shops. Asbestos is the Saturday night special of frictional materials, cheap, unsophisticated and lethal.
So what to do about the problematic brake bands on 3500-lb truck that will soon be careening down Russian hill? One solution is to have the shoes relined by an industrial brake and clutch specialist. These outfits stopped using asbestos years ago and, to judge from my experience with the Houston-based Heitman Company (800-392-3311), offer quality Grey Rock or Raybestos linings and real technical support.
Master Cylinder
For various reasons having to do with cost, respect for the integrity of the vehicle, and skepticism about aftermarket engineering, I gave up the idea of replacing the front drums with disc brakes. All any brake can do is lock its wheels and the OEM duo-servo brakes certainly do that. The boy will just have to stay off freeways is all.
Yet disc brakes have one important safety feature that I didn�t want to forego: the front wheel brakes operate independently of the rear wheel brakes. OEM-style single-circuit brakes are of a piece, so that a leak anywhere takes down the whole system. You have brakes until the fateful moment when you press the pedal and it sinks to the floor. Diagnostically we can say that a massive leak has robbed the system of pressure or that a pinhole leak has finally depleted the reservoir. Emotionally we can say that the sudden loss of brakes is a primal experience, probably not unlike falling out of an airplane.
The OEM master cylinder mounts from the side; newer dual-circuit master cylinders use a flange mount. I ordered a 1" bore dual-drum Ford master cylinder (the same bore dimension as originally specified) and a mounting bracket from Classic Performance Products (714 522-2000). A 15/16" bore would increase line pressure and reduce pedal effort, but at the cost of greater pedal travel. In other words, a smaller master cylinder bore makes brake adjustment more critical, since you can run out of pedal travel before the shoes make contact with the drums.
The new master cylinder includes residual pressure valves, but does not have provision for a remote fluid reservoir. As things stand now, a hole will have to be cut into floor under the seat for access. The bracket appears to be well designed, but could be constructed of something heavier than 1/4" steel plate. During a panic stop, the average man exerts a force of 300 lb on the brake pedal. That force, multiplied by the pedal leverage, translates to about 1 ton on the master cylinder and its bracket.
If the Texas rain ever stops, I�ll install these parts and report how they work.