Did my car originally have power brakes?

[Michael M.]

I've got a 1972 which I've had for a couple of years, on which I'm currently doing a body-off. Anyway, it has standard brakes with a replacement master cylinder.

While cleaning the underside of the firewall today I was suprised to find "PB" written in grease pencil on the drivers side firewall. I know this is one place where options where designated on the assembly line - and I'm assuming that "PB" means power brakes.

So: is there any way to determine if this car originally had power brakes ??? It is a 4 speed / LS5, and I have absolutely no documentation but it seems, for the most part to have most all of its original parts.

[Joe L.]

Re: Did my car originally have power brakes?

Mike------

I doubt that the car originally had power brakes since these would rarely be removed due, at least, to the trouble that would be involved with converting a PB car to manual brakes. However, it's very easy to tell. Look at the firewall area surrounding the master cylinder. If the car originally had PB you will see holes for the power brake booster mounting. It's VERY, VERY unlikley that anyone would ever change this panel even if they converted a PB car to manual brakes. So, the holes, or some vestige of them if the were filled, should remain.

[Tom R.]

Re: Did my car originally have power brakes?

My 78 has "CB" written in grease pencil on the driver's side...yet original window sticker lists AM/FM Stereo Radio...no CB radio!

I see at times where these markings correspond to RPO's and other times raise questions for the owner regarding originally installed equipment. Somewhere I read that as the body shells were assembled and job numbers scrawled...they were off one or two by the time the assembly passed through the paint booths and reached the trim line. Which may help to understand these unusual inconsistencies.

As Joe indicates...most unlikely someone would have changed the booster on the 72.

[Chuck S.]

Re: Did my car originally have power brakes?

Mike, I always understood the "PB" marked on the driver side lower firewall DOES indicate that the car was originally equipped with power brakes.

My 70 coupe has power brakes and it also is marked "PB" at that location. This "PB" mark is also shown on page 253 of "Fact Book of the 1968-1972 Stingray", 4th Edition, by M. F. Dobbins. Dobbins speculates that the mark was placed on the left-hand lower firewall for the brake installer.

If you study the positioning of the chalk marks, they are placed to aid the assembly line workers in quickly identifying the options to be installed. In the case of power brakes, the mark appears only in this one position directly below where the MC or brake booster is installed. In the case of air-conditioning, "AC" is marked in THREE locations because parts related to the air conditioning option are spread from engine compartment to rear storage compartment.

Joe is correct on the holes in the firewall; in fact, if your carpet is out, you will see a large steel reinforcing plate around the steering column, brake, and clutch linkage firewall penetrations. If the car was originally equipped with power brakes, I would expect to find a square pattern of FOUR holes for the power brake booster. If the car originally had no power brakes, the plate should have two holes on a horizontal line for the non-power master cylinder. If the non-power brakes were retrofitted by an owner, I would expect both the four hole pattern, and two crudely drilled holes for the MC. (Look for burrs or deformation on the hole edges.)

Of course, another explanation could be that the assembly line overlooked the PB mark and by the time the inspector caught it, it was too expensive to change. If special ordered by a individual, I expect that they would have built another car for the order, corrected the order copies for non-power brakes and offered the car to dealer(s) as-built. In that case, it would be interesting to see what the tank sticker listed. This is just speculation on my part; maybe John Hinckley knows what GM did in cases like that.

[Michael M.]

Re: Did my car originally have power brakes?

Chuck: I've studied the firewall, the pedal support,the AIM, the archives... and am totally confused.

The car clearly has the "PB" marking. The firewall has 4 small holes - in a square pattern - and 1 large one ( 2 inch diameter) in the middle where the actuator rod passes through. The top 2 holes have studs ( actually just cap head bolts) pertruding through the firewall from the cockpit side - these hold the current nonPB master cylinder to the firewall. There are 2 bolts in the bottom holes that go in from the engine side and bolt to the pedal support. The actuator rod is connected to the top brake pedal hole and the switch contactor is attached via the bottom pedal whole. I do not see any "cruedly drilled holes".

Also wierd is that the pedal reinforcing reinforcing plate in the wiper cavity has four holes and the rivets - but not a single bolt holding it - as if someone has replaced the pedal support and forgot to replace these bolts.

Thoughts ...???

[Chuck S.]

Re: Did my car originally have power brakes?

Uhhmm...Actually, Mike, I think I am the one that was confused, and I managed to confuse you. We're learning together here.

My previous comments were based on my thinking that both system components, MC or power booster, would mount on the firewall such that the actuator rod axis was the same for both systems. This would have required the factory to have installed two different firewall reinforcing plates depending on how the car was equipped; for PB cars, a reinforcing plate drilled in the square pattern for the booster, or for non-PB cars, another plate drilled with two horizontal holes for the MC. That led to my totally erroneous comments about crudely drilled holes done in an owner retrofit of a factory PB car to non-PB brakes.

From your response and further study of the AIM, I now realize that it wasn't necessary for an owner to drill holes in the reinforcing plate; the General provided a universal reinforcing plate that would work for either system. It reduced parts inventory, it was cheaper, it was simpler.

This thinking was confirmed by a closer examination of my car: the large actuator rod hole in the reinforcing plate is actually designed such that the top of the fiberglass firewall hole can be relieved into an elongated oval that would allow the actuator rod to be positioned precisely between the top two small holes of the square pattern. The top two small holes are also slightly elongated, or slotted, which would allow some up/down adjustment of a non-power brake MC mounted there.

At this point, I don't know if the large hole was actually enlarged at the factory when non-power brakes were installed; the AIM (UPC 5, Sheet A3 for 70) seems to show the non-PB actuator rod centered in a large round hole. That configuration would be incorrect if the MC is installed in the top two small holes, but the AIM drawings are sketches rather than manufacturing drawings.

Details of the reinforcing plate and pedal support installation confirming my observations are shown in the 70 AIM on UPC 5, Sheet A2; it should be the same or near there in your 72 AIM. Note that the main view (left side) shows the enlongated large hole I described in the reinforcing plate. In "View A" (right side), bolts in the two lower small holes, as you described, are shown for the standard non-power brake installation.

The bottom line is: I am now convinced that installation of either power brakes OR non-power brakes is a simple bolt-in for the early C3s. On any given car, either brake system could have been been installed, with only minor modifications, at either the factory or later by an owner. With non-power brakes, the MC was mounted using the top two small holes of the square pattern, and two bolts were added in the lower two holes to secure the pedal support to the firewall. When power brakes were installed, all four small holes were used to mount the power booster, and the studs/nuts of the booster secured the pedal support.

The fact that you have a non-original MC, and that the pedal support bolts are missing from the plenum, are probably clues that someone has been monkeying with the brake installation...quite possibly someone other than the factory is responsible for your non-power brakes.

If it were me, I would restore the car to the PB configuration. It will probably cost you a little more effort and trouble, but you have the chalk marks on the firewall to document the PB, and you will have just gained a valuable option.

BTW, this is, in my opinion, a classic case of listening to what the car is telling you, rather than believeing what some maroon previous owner left for posterity. Good restoration often requires these kinds of decisions.