This is a post from Glenn Uettwiller, and as usual he hit the nail right on the head. Kevin
I've gotta tell you, you're right - it ain't that simple. There are many variations to how you can build the system even with "stock" parts, whether you build a 67-68 4-piston fixed-position caliper system or a 69-style single piston sliding caliper. Also, 67 and 68 were available with or without power assist, while 69 was a package setup - all discs became power assist.
I don't know how long you've been on the list, but I know I've detailed the conversion a couple times, and explained the various component's jobs. Maybe I need to check what Geoff's got in the archives for completeness and accuracy, or he can substitute this explanation. This is probably more complete and thorough than any I've done before.
By the way, there were some earlier wheels which were designed before discs were considered, and due to the way the rim and wheel centers are stamped and welded will certainly interfere with the caliper. However, if you get the slightly later wheels, JC for 14x6 and JK, JM, JS and KS for 69 and later 14x7, they should fit just fine.
Brake basics, in order:
Never use copper tubing for brake lines - only steel. Also, all flares MUST be double-flared for strength. If there is a point that must flex between body and chassis, there should be a loop or two to accommodate the flex. Otherwise, metal becomes brittle due to what is called "work-hardening" and will eventually fracture.
There are 2 mounting holes in the brake pedal arm, one for the power booster, and one for the non-assisted cars. There are some slight differences between boosters for A-body (LeMans, GTO, Chevelle, 442, GS) and F-body (Firebird, Camaro) cars. I've actually seen a letter "A" or "F" stamped into the pushrod. There is special hose used for the power booster - 11/32"ID, and rated for vacuum - very hard to find in rolls, but I've found in 18" lengths in packs at PepBoys. It's actually PCV hose.
The master cylinder is actually 2 master cylinders, one in front of the other in one housing. If one system of the brakes should lose pressure, either the forward piston bottoms in the bore, or the rearward piston bottoms on the forward piston. This means the car will at least brake on one axle set using the hydraulics.
The residual pressure valve (there are 2 in drum brake master cylinders, just inside the bore where the lines screw in) are supposed to maintain 5-7 psi pressure to keep the wheel cylinder cup lips sealed in place. You DO NOT want any pressure on the disc brake system (it causes drag, premature wear, and overheating of pads), only the drum brake section. Disc brake master cylinders do not have a residual check valve in the disc brake side, only the drum brake side. One trick for using the old drum master cylinder is to remove the residual valve on the side you've added the disc brake to.
A metering valve (the round device mounted just under the master cylinder on most 67-70 GM disc brake equipped cars) meters the pressure to the front discs, meaning that since discs run zero clearance, and the rear drum brakes must take up some clearance. Method to do this is to hold back the initial pressure to the front discs until the rears start to build pressure, which indicates the clearance has been met. Then the metering valve is over-ridden.
Next component is the differential switch, included in the distribution block. It's also mounted under the master cylinder or just a bit down line from it (at least it is on the 69 LeMans I was working on today) When the system fails as described regarding the dual master cylinder, the resulting loss of pressure offsets the centered valve in the differential switch touching the pin of the contact, turning on the warning lamp on the dash.
The next device, the proportioning valve, does just that - it alters the proportion of braking at the front versus rear axle sets of brakes. A separate proportioning valve would be found in the line going to the rear brakes (not all Firebirds with discs had this - I believe it was only the V8 cars with AC). Method used is to reduce the braking to the rear, since drum brakes actually need less pressure - they are more efficient at braking initially, until they reach their natural limits for heat and the resulting fade problem (also prone to being almost useless when very wet).
GM wised up for the '71 model years by incorporating the differential switch, metering valve, proportioning valve and line distribution block into ONE component. If you knew the proportion was fine for your vehicle in a particular combination valve, this is a great choice. However, most conversion kits are using an adjustable valve to add in line, after the differential switch. That's what I'm currently stuck on for the LeMans, I need to go buy a double-flaring kit tomorrow to install the adjustable valve.
The other issues you haven't gotten to yet are the calipers - make sure they have the bleeder in the correct position. That doesn't necessarily mean they must be at the top, you need to know that the drilled bleeder hole is at the top inside so the air does in fact bleed out. Also make sure the hoses do not interfere with anything at full turn position. The original hoses had little crimped rings to clamp them to the upper control arms. Some later replacements did not have this feature, and if routed well will work fine. The brackets were different as well, where the steel lines met the hoses at the frame, but if routed well, this can be overlooked.
Rotors are generally no big issue unless you're using the 4-piston fixed-position calipers, then you need the RARE original 2-piece units. The original 2-piece rotors were shaped for this, the later one-piece are shaped more like a top-hat, without the necessary recess. The sliding single-piston calipers fit these just fine. I believe there's a special (read that expensive, but cheaper than new old stock) replacement available that accommodate the 4-piston calipers, somebody correct me if I'm wrong here.
Don't forget bleeding basics, such as bench-bleeding the master cylinder before anything else, then starting at the furthest wheel and DON'T have your pedal person pump the brakes or push too hard - just a steady light down pedal, then open a bleeder, close it and holler "up" at your pedal person, leaving a few seconds between strokes. Five strokes or so, then top up reservoir in master, replace lid. Repeat until done. Pumping the pedal furiously foams the fluid, and pushing too hard can lock up the differential switch to one side. DOT 5 is naturally spongy - stay with DOT 3 and remember it has a tendency to remove paint. Wash all spills and splashes away quickly.
