1967 Manual brakes

[Ralph E.]

Re: 1967 Manual brakes

Sounds like the brake system is fine. I would check the brake pads. Are your brake pads semi-metallic ? If they are the ceramic pads or organic pads the stopping will be different.

[Owen L.]

Re: 1967 Manual brakes

Randy, The master cylinder is the same in 1967 for power vs non power brake cars.

It sounds like Randy had a replacement master (which may not conform to stock '67). What is the bore diameter of the '67 versus after GM went to two different sizes?

[Leif A.]

Re: 1967 Manual brakes

It sounds like Randy had a replacement master (which may not conform to stock '67). What is the bore diameter of the '67 versus after GM went to two different sizes?

Owen,
The standard master cylinder for '67 (non-power or power) was the same and had a 1" bore. The optional J56 brakes master cylinder had a 1 1/8" bore.

[Joe L.]

Re: 1967 Manual brakes

It sounds like Randy had a replacement master (which may not conform to stock '67). What is the bore diameter of the '67 versus after GM went to two different sizes?

Owen------


All 1967 are 1" master cylinder bore size. Beginning in 1968 standard brakes were 1" and power brakes were 1-1/8".

[Joe L.]

Re: 1967 Manual brakes

Owen,
The standard master cylinder for '67 (non-power or power) was the same and had a 1" bore. The optional J56 brakes master cylinder had a 1 1/8" bore.

Leif-------

Yes, I forgot about 1967 J-56 having the 1-1/8" master cylinder.

[Ralph E.]

Re: 1967 Manual brakes

Did John Caruso ever resolve his braking problem?

[John C.]

Re: 1967 Manual brakes

Ralph,
I invested in a Motive Products Power Bleeder and went through the bleeding process again. Did not get the results I hoped for, perhaps a slight difference initially, however with this said, the more I drive the car the more responsive the brakes seem to be. This is what Ed recommended in his response. Thank you all for your help and great recommendations.

[Gary S.]

Re: 1967 Manual brakes

I also posted this on the corvetteforum today, but thought it might be of some interest here:

I was not satisfied with the braking performance of me driving my stock L36 coupe with manual brakes, but didn't objectively know exactly how we were performing. 1967 magazine road test from Motor Trend, I believe, gave a 135' 60-0 braking distance.



Baseline 60-0 data on a nice recently paved asphalt country road:

2001 Toyota Sequoia 2WD Michelin Defender LTX M/S tires (800 treadwear): best stop 3.73sec, 160'. (consistent with Car and Driver's 70-0 distance of 204', couldn't find a 60-0). Backed up data with repeats.



1967 L36 coupe, no weighty options, Diamondback Auburn radial redlines, organic Delco pads: 3.98 sec, 169'


So not as good as a twenty year-old 5100# SUV. Barely able to lockup tires even with strenuous pedal effort, I'd say well over 100#. Verified that by putting the bathroom scale on the pedal and pushing hard with the right foot: 115# without high strain. Checked the master cylinder to make sure it was a 1", which it is. Rotors are good, car does not pull left or right. Hoses are relatively new.
Bought some newfangled Powerstop Evolution Sport carbon-fiber ceramic performance pads from Summit, $50x2, turned rotors, bedded pads, retested.



Next, Hawk HPS 5.0 performance street pads. $108x2 from Amazon/Carlisle Brakes:



Then, the Avon Cr6 ZZ tires went on.



Screenshot from the app below. The orange reference line is the Cayman, the blue line the L36 on Avons.




Other cars 60-0 from other magazine tests:









...something seems not right about the lack of easy lockup ability; one of the things that precipitated this investigation. Question becomes about what to do next, if we can believe a 135' stopping distance from 60mph was to be expected on stock tires in 1967. I'd think that better should be achievable with the Avon tires. The 2.93 sec (135 foot) stopping time measured on Avons translates to .89g. The 3.52sec, (153') stopping time on Diamondbacks translates to .79g. Should I be expecting better?
Regarding the system, as I understand it, there is only one possible pedal ratio on the 1967 linkage, I've confirmed a 1" master cylinder, the pedal is quite firm and does not sink, the hoses are relatively new, no leaks, pistons seemed to move normally when changing pads, rotors supposedly spec'd ok, don't believe there is any glazing, thigh muscles don't look atrophied, car does not pull laterally....

From Duke's 5/23/17 post: "The disk brake SAE paper was actually included as a supplement to the Corvette News issue that announced the 1965 models...
...Total brake system leverage consists of both mechanical and hydraulic leverage. Chevrolet did not want to include a vacuum booster with the base disk brake system, and they established of spec of 120 pounds of pedal force for a 1g stop. (Modern cars are probably no more than 40 pounds.) Given the 4.54:1 mechanical leverage ratio, the hydraulic ratio to achieve the spec is about 44:1, and this was used to determine the relative sizes of the master cylinder and caliper bore sizes.

The total ratio of about 200:1 yields pad travel of .005" for 1" of pedal travel



BTW, also tested a 67 L71 coupe with F41 suspension, standard brakes, organic pads, and those fine Kelsey Goodyear Power Cushion redline bias-plies: a little easier to lockup. 60-0mph stopping time was 4.6sec, distance 199'. Yowch.

[Duke W.]

Re: 1967 Manual brakes

A difference in test equipment might be a cause for some of the differences between the old tests and your test, like when the clock actually starts and stops.

I'm surprised the Avons didn't do better, but they develop peak grip with a tread temperature of 160-180 degrees F. It's tough to get them that hot in any kind or normal road driving, but even when cold in mild weather they should outperform any road tire.

I think the solution is to convert to power brakes and maybe power steering. My arms ache after driving these cars. Even Hector is beginning to complain, and he's more that 15 years younger than me.

Last week I rode in and drove a '65 L-78 convertible with sidepipes F-40 on our deteriorating SoCal roads. The engine had been rebuilt with big cam and will not idle stably below about 1000 revs, and I think all the "suspension movement" was chassis/body flex. I might have appreciated it in my twenties, but it was a little too much at my age.

Thanks for a great test report.

Duke

[Gary S.]

Re: 1967 Manual brakes

Ah, excellent point about the road and tire temperature. When I did the runs last week in the evenings, weather was cool and cloudy, ambients around 60 or so, and I only go a half-mile to the "test track".
We're going to get some normal sun and heat later this week so maybe I'll make a couple midday passes.
I do notice the Avons are quite sticky on hot days; I've got a pea-gravel driveway segment. The tires look a bit like sprinkle-coated donuts those days and I have to go real slow awhile to discharge all the gravel!
Definitely considering the PB route. Had done that to my previous L76 coupe and did not regret it. This L36 is pretty original with some minor mods, so some downside to doing another modification.