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Old 03-06-2019, 07:13 AM
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Mustang II brakes

My recently purchased '48 Lincoln street rod, around 4000 lbs., is equipped with Mustang II 9" brakes. They are not just inadequate- they scare me to the point I will only drive it around the block. Closest thing to compare them to is worn out Model A mechanical brakes- hit the pedal, count to 5 and you eventually stop. The car was only driven 300 miles in the last 15 years- sat around a lot.

I plan on replacing these soon, probably with an 11" Granada or similar brake kit. I'm a bit concerned that there may be something else wrong, though. Here's the setup:

1) Typical hotrod dual master with 7" booster (installed under floorboard with original pedal) and red and blue residual valves. Booster seems to work, pedal goes down when engine starts, but due to camshaft, vacuum is only around 10 lbs.

2) 9" Ford rear with usual drum brakes, confirmed working

3) Mustang II 9" discs on front, confirmed working by jacking up wheels

4) new brake lines everywhere

5) seller claims proportioning valve installed, haven't spotted it yet but have not been completely under car

My question to those who have driven a fairly heavy hotrod car or pickup with Mustang II stock brakes- how do they perform? Can you skid the front wheels or make the front end nosedive? Or do they work about like I described above?

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Old 03-06-2019, 07:15 AM
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What is your vacuum?
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Old 03-06-2019, 07:18 AM
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Quote:
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What is your vacuum?

Just added that to my post - only around 10 lbs, due to "Thumper" cam installed in 350 small block.
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Old 03-06-2019, 07:39 AM
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Brakes

I would say that a lack of vacuum is the first problem. If you change to larger brakes (GM 11" metric) the problem will still be there. If it's stock pedal the ratio should be in the ballpark but wonder about the M/C bore side and type of MC (GM or Ford).

Solutions: One way to go is get rid of the bumpy cam that's causing the low vacuum. Second is to convert to hydroboost. Given the car is a little of the heavy side I would go with the hydroboost .
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Old 03-06-2019, 07:47 AM
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Quote:
Originally Posted by 39 master View Post
I would say that a lack of vacuum is the first problem. If you change to larger brakes (GM 11" metric) the problem will still be there. If it's stock pedal the ratio should be in the ballpark but wonder about the M/C bore side and type of MC (GM or Ford).

Solutions: One way to go is get rid of the bumpy cam that's causing the low vacuum. Second is to convert to hydroboost. Given the car is a little of the heavy side I would go with the hydroboost .
So with inadequate vacuum, no amount of pedal pressure will compensate for the lack of vacuum? Not that familiar with hydroboost but looks like it requires replacement of master cylinder and booster. Is there an add on that can generate more vacuum for my existing setup?
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Old 03-06-2019, 08:02 AM
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You can add a vacuum reservoir or canister to increase vacuum reserve

Like mine in the picture !

Jester

https://www.hotrodders.com/forum/att...1&d=1551884465
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Last edited by painted jester; 03-06-2019 at 08:20 AM.
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Old 03-06-2019, 08:06 AM
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You can add a vacuum reservoir or canister to increase vacuum reserve

Like the one in the picture !

Jester

https://www.hotrodders.com/forum/att...1&d=1551884465

Thought about that, but was told that it will still peak out at 10 lbs vacuum- will only keep it from getting even lower at times. I see Summit has auxiliary vacuum pumps that would probably do the trick, but I'd probably rather have a milder cam with a smoother idle if I can find a shop I trust to install it.
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Old 03-06-2019, 08:18 AM
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As RPM increases your vacuum increases with a large overlap cam with no vacuum at idle the canister stores this higher vacuum and more volume! MY CAM IS .613 LIFT AND 312 DURATION 4 DEGREES RETARDED !! ALMOST ZERO VACUUM AT IDLE AND THE CANISTER WORKS VERY WELL ( sorry hit cap.s by mistake) without the canister I would have almost no brake !!!

Just make sure you get a quality reservoir!! vacuum pumps work very well also!

How big is your cam
Thumpers arn't that big
Jester

Last edited by painted jester; 03-06-2019 at 08:34 AM.
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Old 03-06-2019, 08:21 AM
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granada calipers

The Ford design sliding calipers tend to rust and not slide, wear out one side of the rotor and pad, the Gm pin calipers work better but volumn is not matched. the new Mustang Gt dual piston with the GM design pin mounts work best. Some of the mustang -smaller ford forumns show how to make the Gt adaptor brackets.
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Old 03-06-2019, 08:32 AM
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Quote:
Originally Posted by timothale View Post
The Ford design sliding calipers tend to rust and not slide, wear out one side of the rotor and pad, the Gm pin calipers work better but volumn is not matched. the new Mustang Gt dual piston with the GM design pin mounts work best. Some of the mustang -smaller ford forumns show how to make the Gt adaptor brackets.
I was already looking at dual piston calipers but they are very limited in the ready to go kits and I'm not able to fabricate. How about this kit on Ebay? https://www.ebay.com/itm/Mustang-II-...IAAOxyyq5TNIkL
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Old 03-06-2019, 08:54 AM
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A slow turning engine will produce less manifold vacuum than a fast turning engine. Manifold vacuum will be at its highest when the engine RPM is higher, and the throttle plate is not open all the way! If an engine has low compression manifold vacuum will be low during the cylinders intake stroke. Thumpers need high compression to get all its potential installing a thumper on a low compression engine just to make it sound like its BAD A S S is only asking for problems!!

Low or poor sealing rings will drop compression and vacuum !

A vacuum reading at idle that is much lower than normal might indicate a very large cam overlap but leakage through the intake manifold gaskets, manifold to carburetor gaskets, vacuum brake booster or the vacuum modulator leak will also. Low readings could also be caused by very late valve timing or worn piston rings.

These should be checked before jumping to a conclusion

Jester
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Old 03-06-2019, 09:13 AM
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Most big cam early muscle cars had manual brakes because of low vacuum ! Power brakes or vacuum assist was created years ago for women who had less leg strength then " big masculine men " LOL NONE of my drag cars had power brakes LOL

Even with only rear brakes and spoke wheels on the straight axles my drag cars or hot rods stopped very well with manual brakes!

Jester

Last edited by painted jester; 03-06-2019 at 09:25 AM.
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Old 03-06-2019, 01:30 PM
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Quote:
Originally Posted by painted jester View Post
Most big cam early muscle cars had manual brakes because of low vacuum ! Power brakes or vacuum assist was created years ago for women who had less leg strength then " big masculine men " LOL NONE of my drag cars had power brakes LOL

Even with only rear brakes and spoke wheels on the straight axles my drag cars or hot rods stopped very well with manual brakes!

Jester
I remember my dad's '69 LTD with manual brakes well, had no problem stopping with them and felt the other US cars of the era with power brakes had too much assist. Imports seemed about right.
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Old 03-06-2019, 04:50 PM
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I am of the opinion that manual disc brakes with the proper pedal ratio is the hot tip (start at 7:1 and alter to suit).
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Old 03-06-2019, 05:42 PM
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Quote:
Originally Posted by painted jester View Post
As RPM increases your vacuum increases with a large overlap cam with no vacuum at idle the canister stores this higher vacuum and more volume! MY CAM IS .613 LIFT AND 312 DURATION 4 DEGREES RETARDED !! ALMOST ZERO VACUUM AT IDLE AND THE CANISTER WORKS VERY WELL ( sorry hit cap.s by mistake) without the canister I would have almost no brake !!!

Just make sure you get a quality reservoir!! vacuum pumps work very well also!

How big is your cam
Thumpers arn't that big
Jester

I think it's a mild Thumper. At one point (trying to get it timed right, etc.) I had the engine idling fairly smooth but right now it's loping a bit. The reservoir seems like an easy thing to try without changing the booster, etc. I'm also going to check the discs closer- one drags and one is more free when turning the wheel- and make sure the caliper is sliding freely, etc.
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