Front disc upgrade for '69 SS w/ rally wheels - Impala Tech
Brakes & Suspension Conversion Questions & more

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post #1 of 6 (permalink) Old 05-26-2015, 02:28 PM Thread Starter
Join Date: Nov 2011
Location: Seattle, WA
Posts: 8
Front disc upgrade for '69 SS w/ rally wheels

I have a long and horrible story about terrible braking that's been through a frame-off restoration and two talented shops. I'll spare you the extensive details, but I'm now going to go through and systematically upgrade parts until things are right. In short, the brakes are slow but ultimately powerful. If you stomp/kick the brake very little happens. If you apply slow stead braking, it's great. There's tons of assist, it's just slow to respond and the brakes can only be applied so fast - and not fast enough for an important or panic stop. Stick a gauge on it, plenty of pressure, you just can't get pressure fast.

It's hard to explain without driving it. On a test drive the shops will demonstrate that there's ample braking. I'll say "Great, no panic stop". And it won't be able to, and they'll say "That's how old cars are", but I have a bone-stock front-disc '69 Convertible that has spectacular brakes (for 1969). That's all I want: factory braking. If I can get better, great. I also have a '69 Camaro and a '70 GMC for reference, and they are an order of magnitude better. This was originally a drum/drum car and I uprgaded to front and rear disc from MP Brakes at the same time, so I can't tell if it's the front or rears, but the fronts should be quick to respond even if it didn't have rears, so that's not it.

It's genuinely unsafe and I don't drive it as a result. It's sat there post-frame-off for 6 years and it's time to compromise and give up stock underhood appearance if needed to make it safe and fun.

Anyhow, I currently have front factory discs, rear GM discs (those integral park brake Cadillac units) and a '69 Corvette MC. I think that's a mismatch as the factory MC bore should be 1" and the Corvette is 1 1/8th, though not sure if that's enough to make a huge differences.

In any event, I'm wondering if anyone makes a front and rear disc upgrade for these spindles (or replacement spindles if necessary) to fit a '69. I believe 65-70 are similar and I've looked at the '65 thread but am not sure.

I ordered a Wilwood MC and distribution block to start there; if that's not enough, I'd like to move onto the brakes themselves.
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post #2 of 6 (permalink) Old 05-26-2015, 04:17 PM
Join Date: Jan 2006
Posts: 12,458
Did you install an aftermarket cam? You know which cam! The one all of the cool people have. The cam with a rough, rumpitty-rump idle that everyone else has? If so you have found your problem. To get stock brake performance you will have to go with the solution the Chevrolet engineers came up with to handle the same braking problem in a Chevy pickup with a diesel engine installed under the hood.

Diesel engines with power assist brakes come with a Hydro-Boost. The Hydro-Boost eliminates the big eleven inch in diameter vacuum powered booster diaphragm that is stock on all power assisted brake cars (with or without disc brakes).

Stock Eleven Inch Power Booster clearing a Big Block Chevy with stock valve covers:

A SBC with n eight inch power booster:

Of course if you have a smaller diameter power assist vacuum diaphragm to clear those really cool tall valve covers that are not needed on a car unless you have a stud girdle sitting on top of your rocker arms: that could be part of your poor brake performance as much as a cam with no manifold vacuum.

This all assumes of course that you even have power assisted brakes to start with. If you have manual braking, yet it is slow to act with good brake line pressure (in the range of 800 to 1,350 psi without stomping on the pedal with both feet) then I suspect your master cylinder was not completely bench bled. As such it will have a little bit of entrapped air inside the cylinder bore.

Bench Bleeding a Master Cylinder:

Entrapped Air in the Master Cylinder:

However if you have driven three to four thousand miles since you restored the car (and replaced the brake master cylinder) then any trapped air in the MC should have been released by now. Your problem could be mismatched parts since there were four different Chevrolet car master cylinder, internal bore cylinder sizes that ranged from a small 3/4 inch up to a 1-1/8th inch diameter. According to Pascal's Law (how hydraulics works) if you change one item it affects how the other components work.

Big Dave
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post #3 of 6 (permalink) Old 05-26-2015, 07:35 PM Thread Starter
Join Date: Nov 2011
Location: Seattle, WA
Posts: 8
Thanks, but no, per my post, assist is not the problem. I do not have a big cam. I do not have tall valve covers. I have already tried a hydraboost to eliminate that as a possibility. Trust me, all of the basics have been checked by myself, Musclecar Restorations, and two premiere local classic vehicle shops (one specialized in Corvettes, but its a Corvette MC, so why not). It's a defective part or a complete mismatch of parts, but the vendor assured me the latter is not the case, and you'd think others with the standard Master Power Brakes conversions for these would have the same problem if it were endemic to the package itself.

BTW, an easy way to rule out low vacuum is simply to brake while coasting down. You can make 25 inHg even in a radical car when engine braking, and the vacuum reservoir will hold that high vacuum for a little bit. Easy test.

Air would make the pedal spongy I imagine, not slow. It could, as you note, make the pedal feel slow because you compress the air before you move the fluid, but the pedal is high and hard, rock solid.

I did bench bleed it. Technically I can't be certain the last person to touch it did, but it was the same before when I had, so can't really be it. Plus it'd be spongier. I put the bore sizes and the expected bore size for the front pistons above.

I didn't want to mention it, but this is actually a Canadian-built Pontiac 2+2 427, but it is mechanically -identical- to the Impala and has no Pontiac running gear. So please just think Impala, nothing to do with Pontiac. That's why it's a 427 and not a 428. 100% Impala, same wheelbase, same drivetrain, same chassis, suspension, brakes, everything - same assembly line.

Random tidbit: On that "Trucks" TV show they're converting an old C10 gas pickup to Duramax. They kept talking about trying to save the booster but clearance was an issue. Is there ANY role for a vacuum booster in a Duramax diesel truck??? It made no sense to me.

So, back to my actual question...
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post #4 of 6 (permalink) Old 05-27-2015, 10:24 AM
Join Date: Jan 2006
Posts: 12,458
Random tidbit: On that "Trucks" TV show they're converting an old C10 gas pickup to Duramax. They kept talking about trying to save the booster but clearance was an issue. Is there ANY role for a vacuum booster in a Duramax diesel truck??? It made no sense to me.

No. A diesel engine controls throttle speed by varying the amount of fuel that enters the engine not the amount of air. There are no throttle blades other than that to shut down the engine entirely (throttle it as in choke it death) because the intake manifold is open to atmospheric pressure. Since there is no manifold vacuum there is nothing for a vacuum booster to work with. Hence the Hydraboost which uses a differential in the hydraulic line pressure from the power steering pump to activate the brakes.

The only thing left are the control valves in the hydraulic circle. Disc brake cars have a residual vale that holds the pads close to but not touching the rotors as the volume of the wheel cylinder is so large that it would take time to pump it up if the pads where fully retracted (and a lot of leg movement with the pedal going to near the floor).
There are proportioning valves on disc/drum systems that allow the front brakes to apply before the rear brakes. But if you delay too long a time and you could hit something before the rear brakes ever fully applied. Both of these valves are sized to match the components (you can buy a one size fits all adjustable proportioning valve but then you would have to adjust it and lock it in place to keep helpful participants at car shows kids asking wonder what this does?

Big Dave
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post #5 of 6 (permalink) Old 05-27-2015, 12:02 PM Thread Starter
Join Date: Nov 2011
Location: Seattle, WA
Posts: 8
So how about the front disc package for a '69 with rally wheels? Anyone know of anything?

What about fitting F-body parts? There's a billion options there if we could somehow use their spindles, but I presume they're completely wrong and probably too short.
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post #6 of 6 (permalink) Old 05-27-2015, 01:47 PM
Join Date: Jan 2006
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There were two different Rally wheels foffered in 1967-69. One for drums and another for disc brakes on a Camaro in 1969 (first year the new one piston full floating caliper was offered). This was because the Camaro, Chevelle, and the Nova that had these brakes as an option in 1969 used a 14 inch wheel. By 1970 the wheels offered with disc brakes had jumped to 15 inches due to the number of complaints about torn tire beads caused by the lack of a bead well on disc brake steel wheel. Virtually all 15 inch wheels will clear a full floating single piston caliper as used by GM.

Most of the kits you see offering disc brake upgrades for the 1965-'70 Impala use the disc brake splndles and calipers and rotors off of a 1981-'88 Monte Carlo or Malibu/El Camino. The spindles are redilled from their metric bores and threads to accet being bolted onto your English size and taper ball joint Impala (they do not directly interchange without machine work). Of course the brake upgrade kits use offshore components of questionable quailty chosen because they are the cheapest parts the vendor could find to put in his kit, not the best parts.

The difference between the factory original parts and the Monte carlo parts used oin kits is in the size of the components. The Chevroltey engineers were woried about the big car being able to stop with the smaller brake parts used on the smaller cars. So 1969-'70 brakes off an Impala and light truck were larger (piston size and actual caliper dimensions). In 1971 Chevy decided that the smaller brake parts would work and standardized their fleet on those parts for the Impala line (keeping the bigger caliper for use on light trucks). Unfortunately the spindles do not interchange between the 1971-'76 and the earlier 1965-'70 body styles.

You can use the earlier Corvette fixed bridge four piston calipers and thicker rotors that were installed on 1968 Impala's. Those parts will fit the 1965 spindle, but it requires a special one year only hub to attach the rotor and disc brake spindles (more properly called a steering knuckle as there is more than just the spindle on this part).

Big Dave
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