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Marlin Brake Booster

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Heavy 488 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: May/06/2021 at 7:11pm
If he has marginal brakes now, size up is the wrong way to go.
Either way, playing with master cylinder sizes to patch a failed element in the system is a bandaid
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: May/06/2021 at 7:28pm
Sounds like you are on track to send in the booster you have, if it is likely from a drum brake '66.
Normally a disc brake booster is dual diaphragm, and a drum brake booster is single.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: May/06/2021 at 10:04pm
I'm with Heavy here. Is the issue solely pedal pressure? That was kind of implied int he first few posts. Does it brake well, and evenly, except for excessive pedal pressure? I assume so, but thought I'd ask...

Consider that it could have been wrong before you or your dad ever owned it. 

Mixed or wrong parts is all too common. Having done a lot of experimentation with master cylinder bore sizes, too big ot too small will increase/decrease pedal pressure, it has no other effect. And with a working booster, it won't matter that much. And when it does matter, it is smoothly so. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: May/07/2021 at 8:11am
Looking at the part numbers for the master cylinders, 3208311 power drum brake and 3207351 for power disk brakes, they both have 1" bores so my earlier suggestion is as others stated, not worth going there.
I would however ensure the master cylinder does have residual check valves in both ports. I have received new/rebuilt and they have been forgotten or even installed when they should not. Without them, you will have poor drum brake performance regardless of the booster condition.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/10/2021 at 7:41am
I've run disc brakes on the drum master cylinder on several different cars, on for over 10 years. Works just fine once the residual pressure valve is removed from the front outlet of the MC. The main difference in a drum or disc MC is that the front (disc) reservoir is made to hold enough fluid for the life of the pads even if you never check fluid levels. That was an idea to "dummy proof" the system by engineers. Modern cars don't do that any more. The chances that no one checks the fluid over the 40K or so life of the pads is slim to none. The larger reservoir is not necessary!

Typically, drum power brake boosters are weaker than disc boosters. In the 70s AMC used single diaphragm boosters on drum cars, dual diaphragm (or larger diameter, sometimes dual and larger) for discs. Drums have a "servo action" (shoes twist inside drums, making them grip tighter), discs don't. I've used drum boosters on disc conversions and actually like it. You get some extra power to the MC, but not as much as the typically over boosted (IMHO) 70s cars. A good compromise between the factory disc booster and manual discs. Manual discs are ok on a small light car (I ran them on a 63 American), but I wouldn't run them on a full size car, or anything you might want to bring down from high speed quickly -- though I could do so on my 63 American. It would lock all four tires, but I had the wide power brake pedal. "Power brakes" was when I put both feet on the pedal and held my a** in the seat with the steering wheel...  that would bring it down pretty quick, and lock all four if street was wet or on dirt/gravel!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jlswaggers Quote  Post ReplyReply Direct Link To This Post Posted: May/15/2021 at 8:26pm
if originality is not a problem, you can use a mopar booster.  i used one from a 77 volare on my 68 rebel with drum-drum brakes. made a world of difference.  better brakes than when new
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: May/16/2021 at 2:46am
Specifying the year 1966 rather than 1965 helps prevent getting the wrong brake parts for a 1965 Marlin that has four wheel drum brakes. Technically, all 1965 Marlins had front disk brakes, at least that's how the parts catalogs will see it. 1966 Marlins came either way, with four wheel drums standard. 

Disk brake cars use entirely different rear brakes too. You do not want to have a mismatch of brake parts. Few parts that are for a four wheel drum car will work properly on a front disk brake car, or vice-versa. 

My 1966 Marlin has 4-wheel power drum brakes (thank goodness) and the booster diaphragm ruptured once. It creates a massive vacuum leak when you push the pedal that will nearly stall the engine at idle. It's also a 100% loss of boost, and I can tell you it took nearly all my strength in my right leg to stop that car without boost. Driving a power brake car with a failed booster is MUCH harder than driving a manual brake car. 

I can also tell you that when working properly, those four wheel power drums can put you through the windshield of that Marlin with light pressure on the pedal. They work extremely well. 
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1966 Marlin
1972 Wagoneer
1973 Ambassador
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1982 Concord D/L
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