Okeh, so we know a smaller master cylinder bore means an easier piston push over a longer stroke, and a larger master cylinder bore means a harder piston push over a shorter stroke.

So it makes sense that a 15/16" master cylinder would be specified with nonpower brakes, and a 33/32" (1-1/32") master would be specified with power brakes: Without a booster, the smaller bore makes it easier to push the pedal.

So why would Chrysler specify a 15/16" master with power disc brakes in '73-'75, and a 1-1/32" unit with nonpower discs? :shrug: The '76s got 1-1/32" with power or nonpower, and the power booster grew larger as well; probably in conjunction with the increased disc brake caliper piston size.

(I'm trying to decide which size master cylinder to get for my '73)

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

They may be using different leverages on the pedal.........

I know GM had different push-rod attachments power vs non-power.

(the push-rod attachment is closer to the top pivot on non-power so the ratio of [pedal-to-pivot / pushrod-to-pivot] is greater on non-power)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Hey I'm about to buy a booster and master cylinder for my 64 barracuda and noticed a difference between the 1976 and the 1973 booster. I have set up my car with the 1973 type slider disk. The 1976 type of booster and MC runs about $15 cheaper at autozone then the 1973 and the core charge is also $10 cheaper so I could save$25 by going with the 1976. I just do not know if there is a difference in the mounting and the push rod,or anything else I may be overlooking.

Joe

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1964 barracuda 198 slant

The only difference there is size ('76 booster is larger). All mounting is the same.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I know that the 1973-1975 Calipers had a smaller bore size than the 1976+ brethen if I remember right, did they think that they needed a bigger bore MC because the smaller caliper pistons then decided to reverse things??? It kind of doesn't make sense to me either...



-D.Idiot

No idea why they specced the smaller MC, but I have always used the 1 1/32 master with the later piston diameter. I have used the B-van 1 1/8 aluminum master too with good results. 15/16 seems like it woud require just too much travel. The flex in the factory pedal assembly does not help things.

Maybe the answer is in the travel. Perhaps the factory pedal assembly does not have enough available travel to use the 15/16 master. The power brake assembly DOES have enough travel due to the pedal ratio (also since the force at the pedal is lower it means less flex of the pedal assembly).

I don't know about you guys, but with the larger front calipers and 1 1/32 master, rubber lines, in my '64 I can ALMOST put the brake pedal on the floor by standing on it with all my strength. I bet I would be able to with the 15/16 master.

Well, I dropped the NOS booster off at a rebuild house in Spamilton (Hamilton) today. Turns out they also do stainless-sleeved master cylinder rebuilds. Maybe I'll go ahead and buy the 1-1/32" new master and have the 15/16" master sleeved and rebuilt...that way if the larger item is a disaster (which frankly I doubt) I can swap the smaller one back on.

I'm also going to swap the 15/16" stock rear wheel cylinders for new 13/16" items to get rid of the (in)famous disc-brake-A-body easy rear lockup. I cannot imagine why Chrysler decided only cops should have cars with rear brakes that don't lock up

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

My '75 Coronet with bone stock non-power disk brakes stopped very well, never missed power brakes, but I don't remember to MC size, sorry. It just seemed to work fine.

That's the key. generally, nonpower cars had 4:1, and PB, sometimes via external bellcrank, were around 2.5:1.

This is really a complex subject. The one serious problem you can run into with power brakes is booster runout. This occurs when you've "used up" all the boost, and the booster bottoms. Back in the '50s, this was hyped as a safety feature "even if the power brakes fail...", which is true. But when that happens, and you have 4:1 with a big bore, it can really be a bear to stop a disc car. Duo-servo drums (which are, in my opinion, the worst thing to come out of Detroit....just try to panic-stop without lockup!) are passable with a dead booster. (They are self-energizing to the max.)

Most factory PDB cars had either a very large booster, or a dual (tandem) booster, either of which increase line (for the same pedal pressure) and/or forestall runout.

The problem got even more interesting when the Feds initiated their pedal-pressure rules in the late '70s.

Personally, on any car under about 4,000 lbs. (unladen), even with discs, I prefer no booster and a large bore master. Clearly, not everyone agrees, and this may not be suitable for 90lb. persons. For my dime, however, being able to threshold brake under panic conditions is worth the leg-muscle-building effort. If you'd like an explanation of threshold braking, see: http://www.moparaction.com/extra/Driving/Driving.html

Now I'm gonna get a bit off topic. Whatever kind of brakes you have, be sure to precisely dial in the F/R proportioning. Correct vs. "factory assembly line compromise" or "I bolted it together with this 'n' that" has a profound affect on stopping distances and car control. And remember: You want the fronts to lock first (by a hair's breadth). If the fronts are locked, a car can go straight (not be steered, just go straight). With the rears locked, it MUST spin!

Rick

Welcome to the board Rick,

Your front / rear brake bias rules are probaly very valid if you live in California and drive on dry freeways.

In my part of the world you would slowly run the stop sign with your front brakes locked and your rear wheels pushing them at a high idle......every time it snows.

More balanced braking is a good choice for other situations.....like rain, snow, low speeds, gravel.

Further off topic...The best brake bias setup on my 65 Valiant with front late A discs and Explorer rear discs was achieved with a factory 75 combo valve.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Anyone know a good place/brand to get a large bore master cylinder?

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"You know a car is really yours when you've touched every fastener on it." -David Freiburger

Welp, I've got my new master cylinder (1-1/32" bore) and my rebuilt booster. Just gotta find the combination of time, daylight, and mood to swap them in. I have a nice set of the '76 (2.75" calipers, but the present '73-type units (2.6") are working fine so will stay in place for now.

I've been driving around for three or four weeks with the dead booster on the car but no vacuum line running to it. Even with this grossly reduced mechanical advantage over the brakes, it's still way too easy to lock up the rears, so the 15/16" rear wheel cylinders will be swapped off for new 13/16" units. If I find I still need to shift more brake bias to the front — which my experience with other disc brake A-bodies tells me isn't terribly likely — then I'll swap on the bigger-bore calipers.

I have to admit it is tempting to delete the booster, even if for no other reason than to be able to use an air cleaner with the snorkel pointing outward-forward rather than inward-rearward (pointing at the passenger side of the firewall). Between the A/C compressor, the dipstick tube, and the master cylinder's location in front of the booster, the '73s ran out of room for the regular air cleaner and this was Chrysler's solution.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yes. Please start a new thread.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

This brings up another very important point. It is imperative to get the F/R proportioning as close as possible with NO valve, then use an adj. valve for that last few percent. Why? Because all prop valves have a "cut-in" point, usually 300 PSI or higher. This means that they do nothing on ice, snow, and, especially, in the rain, where you can be traveling at high speeds and really don't need a locked wheel!

Juggling real wheel cylinder bores vs. caliper bores is the way this is done. Great point!

You still want a prop valve, though, because F/R bias requirements vary with speed. High speeds = harder braking (more line pressure) = greater R to F weight transfer = higher propensity for the rears to lock!

Rick

Rick, just curious: Why are you typing extra-loudly like that (bigger font size)? The regular font size is entirely legible. Just sayin'.

I probably will not be installing an adjustable prop valve. I have never needed one on a disc-braked A-body in the type of driving I do. The last car I set up had the pre-'73 4-piston front discs, 10" rear drums with 13/16" cylinders, and a 1" master with no booster. Its system balance was very nearly perfect, whether I was dealing with icy Denver streets or hauling down from 80 mph on a hot, sunny day.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.