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disc brake conversion

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cc gremlin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cc gremlin Quote  Post ReplyReply Direct Link To This Post Topic: disc brake conversion
    Posted: Sep/03/2012 at 7:01pm
converting my 73 gremmy to discs  which I already have installed however I am a little clueless on what master cylinder to use I do know 3 were offered on that car but Im sort of looking for a part # also would a wagoneer or jeep master bolt up (originaly the car had drum brakes with power booster) Thank you In advance Chris
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Post Options Post Options   Thanks (1) Thanks(1)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2012 at 9:06pm
You could probably use the original Power drum brake master and booster, if you remove the residual check valve from the port going to the front brakes.

'73 Gremlins also could be optioned with manual drum brakes. You can find a new version of that MC. 

Someone else can inform you on the whether a Jeep one will interchange.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote jcisworthy Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2012 at 6:24am
When I convert my cars I use a disc brake master cylinder master for the vehicle. I think the fluid capacity is larger than the drum brake master but not positive. It was always a bolt up change over. I actually have a GM master on my 68 Javelin right now which is a bolt up with the right rod from the brake pedal to the back of the master and it has an 1 1/8" piston which makes for less pedal effort to stop with the manual disc brakes I have compared to the manual disc AMC set up. Lines come out on the opposite of master so some plumbing is necessary but it works well. 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote FuzzFace2 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2012 at 9:18am
Thing is the power option I dont know if there was a power disc option but if there was you should be able to get that master and bolt it up to the booster.
Most of the disc options were non-power and I dont know if that master would bolt up to the booster or not?
As said you could just pull the valve out of the drum master and use it as said above.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2012 at 9:57am
Do not use your drum master. By nature of their design, disc brakes require more fluid to operate, especially the large single piston discs. Any late model Jav or Hornet master should work. Likewise, the disc booster has a dual diaghram whereas the drum is a single diaghram due to the pedal effort being greater with discs over drums. A single diaphram booster can be used can be use discs but will result in a harder pedal.

Any disc brake booster from an American/Rogue or Hornet will work on your car. It is the same booster used on the SC/Ramblers as well.
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2012 at 10:39am
While I'd agree with Dan, capacity.... I've also seen factory disk with a master that is 50/50 rather than what's closer to 60/40 for disk. 
The large piston does indeed take more fluid - but one argument for using a 50/50 drum (of the correct PISTON size) is that once you start to push the pedal, the reservoir is closed off from the system, so all you get is what's in the cylinder bore being pushed by the piston.
In other words, once the foot starts to push the pedal, the master reservoir size no longer matters. However, if for some reason the piston was retracted just a tad more, say a rotor that's not true, out of parallel, etc. or a loose wheel bearing, then you would want the extra reserve as the second application of the brakes is going to pull more fluid in, fluid you may not have if it's a bit low already. It's a safety factor as well as filling the need for extra fluid. 
I would also add it's partially dependent on the size of the master cylinder as well - some simply hold more fluid. I've seen some drum brake masters I'd never use on disk as the reservoir is simply too small.

In ANY case, no matter what, make sure there is no residual pressure check valve in the outlet you use for the disk. A disk master won't have it - but if you go cheap and get a reman, DO check. Those fools slap parts together, and don't know or care about application during assembly, and in some cases, things are boxed or marked incorrectly, or are a "will-fit" application intended for 4 wheel drum or disk/drum applications.

Because I'm a real stinker on safety, including YOUR safety, my preference is to use the correct master intended for disk brakes and be done with it. No guessing; no "gee, I wonder why the brakes didn't work when I REALLY needed them on that rough curve back there" as you slam into the side of a soccer mom's van full of kids.  Wink

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Post Options Post Options   Thanks (0) Thanks(0)   Quote cc gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Sep/05/2012 at 7:51pm
Thanks to all for your opinions I  have to dig more  into this I should have also added the fact Im sizing up a mustang rear end with discs maybe adapting the whole ford assembly(master and boost ) might be an option or of course there is always the boat anchor and rope
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/05/2012 at 9:40pm
Dan is partially correct. The small 2.6" piston Bendix and Delco calipers, as well as the 2.75" piston Kelsey-Hayes calipers, can be used with a drum master cylinder once the residual pressure valve has been removed from the front brake outlet. The 3.1" piston Bendix calipers (only used 75-78 -- 75-76 on all, 77-78 Matador only) need a larger bore MC... 1.125" IIRC.  Drums and the smaller calipers all use a 1" bore MC with approximately the same stroke. I've used the drum MC with the small calipers for many years on several cars with stellar performance.

I adapted a Ford Ranger booster an MC to my 63 Classic. The Ranger MC is a disc/drum MC but I have four wheel discs. My rear discs are 86 Jaguar dual piston (one on each side of the rotor). The Ranger 8" dual diaphragm booster doesn't have quite the power of a typical 60s/70s American disc brake booster, but a little more than the single diaphragm 63-66 Classic drum booster (which I ran until it failed).  I like having a little more brake feel while not having to push real hard to stop.


Edited by farna - Sep/05/2012 at 9:42pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx39068 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/05/2012 at 11:15pm
Exactly what part was incorrect Frank? AMC used a larger resevoir for discs on the master than for drum brakes regardless of the type of disc brakes, fact not opinion. AMC used the larger dual diaphram booster for disc brakes compared to drum brakes, fact not opinion. The piston in the single piston KH caliper is huge when compared to the dinky little pistons on the earlier Bendix system, fact not opinion.

When you say "paritally correct' it would be helpful to all to see the facts that you are basing your comments on rather than your opinion.

Furthermore, the comments you make about your setup have nothing to do with what the OP asked about and in particular your comments regarding Ranger MC and Jaguar rear discs are irrelevant to what he was asking about. For his applicagtion he needs to use the master and booster that was used on the Jeep or Wagoneer brake setup came from.

Regardless, do you really think that all engineers at AMC were a bunch of know nothing idiots for designing the larger booster and big resevoir master all because you have had "stellar performance" while using the smaller calipers and rotors on your small Americans?

The OP was asking about using AMC parts from AMC vehicles that the AMC engineers determined needed the larger MC and dual diaphram booster so your answer was both misleading and potentially putting the OP at risk of having substandard brake performance with the parts he was inquiring about.


Edited by amx39068 - Sep/05/2012 at 11:17pm
Dan Curtis, AZ AMC Collector Quality Restorations & Parts - amcmusclecars.com
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Sep/06/2012 at 6:59pm
I thought I was clear, but will address a couple things you brought up.

"Partially" correct was pointed out -- the drum MC will work fine (with the small calipers),  you stated the drum MCs weren't usable.

I pointed out what I was using to illustrate that there are options. If you don't understand hydraulics and brake systems you shouldn't do any modifying. That's usually understood, but maybe I should have stated such.

The reservoir is bigger for the disc portion in an effort to idiot proof the system. It has no effect on brake performance at all. The larger reservoir holds enough fluid to last the life of the pads and still supply brakes even if you never check the fluid level. Cars made over the last 10+ years have abandoned this design and use same size and even partially shared reservoirs.  It's the bore and stroke of the piston that determine braking power.  Billd more or less pointed this out.

The larger booster used with disc brakes is there to make the brakes easy to apply. AMC also made disc brake systems without boosters. Boosters aren't needed at all -- how much boost you want is pretty much a personal preference, or a physical necessity in some cases. I don't think non-power discs were available on the big V-8 cars, but they were on Javelin and small cars.  The big cars may indeed need boosters for the disc brakes, haven't researched that.  The big drum brake cars did come with manual brakes (no boost).

Not having boost (or enough boost) could mean longer stopping distances.  Part of that depends on how much effort you're physically capable of, part of it is in the pedal ratio.  Boosters are made so that even a small person can have great brakes, and are necessary even for big strong legged people in heavy vehicles.

I've talked extensively with the experts at Master Power Brakes. This was maybe 5-6 years ago when researching a major AMC brake article.  I understand your concern for safety, but would appreciate a bit more of a professional attitude when replying to something I've posted that you think might be misleading or in error.


Edited by farna - Sep/06/2012 at 7:00pm
Frank Swygert

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