EARLY A booster

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Valkman

Valkman

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Hey all, tried the search thing and there's just too much to go through. I currently have a 73 manual disc setup on my 65 Barracuda. The car stops great but it has a rock hard petal and I'd like to soften it up a bit. I have 2 questions:
1st is there a difference in master cylinder force (piston Dia.) and can I tell what I have.
2nd what booster would work and what (if any) modifications would be needed to add one?
 
You would need a master cylinder with the bore diameter best for power brakes. Might be able find your bore with you parts number. You’ll need a power brake booster adapter.
 
install any 73 up booster off any ABEFMJ car line, that has the same mounting pattern. Then you'll probably be wanting to make it easier to modulate which just a matter of swapping reservoirs.
Before you reinstall the MC, measure the bore diameter to the nearest 1/16th inch.
Caution: make sure the pushrod is NOT too long, or you could break the control valve on the first hard stop.

But before you do anything;
make sure the rear brakes are not dragging. And make sure your current MC is correctly plumbed; the frontmost reservoir should go to the rear brakes.
 
If your brakes are really that hard, you have something wrong. Drum brakes have hard pedals when they are right, but discs always have a little "give" even with manual discs. As light as the early A bodies are, I would never convert to power brakes. I think your solution is elsewhere. Maybe another master cylinder or proportioning valve.
 
RustyRatRod said:
If your brakes are really that hard, you have something wrong. Drum brakes have hard pedals when they are right, but discs always have a little "give" even with manual discs. As light as the early A bodies are, I would never convert to power brakes. I think your solution is elsewhere. Maybe another master cylinder or proportioning valve.
Click to expand...

Yeah I've owned a 66 73 Barracuda, and 65 Fury all with manual brakes and I don't remember having issues using them, but then again I was a lot younger then! That's why I wasn't sure if there is a difference in master cylinders and that would have something to do with it.
 
AJ/FormS said:
But before you do anything;
make sure the rear brakes are not dragging. And make sure your current MC is correctly plumbed; the frontmost reservoir should go to the rear brakes.
Click to expand...
I thought the small reservoir(front) was for he disc brakes. I'll have to check out to see how I actually have
 
early A used a small Bendix diaphragm booster and linkage like this one with a reservoir tank, this is only $187 shipped. Not bad considering the booster itself is $153. Unknown MC bore diameter. Make sure you got the disk/drum prop valve too.

g800110_1.jpg

Right Stuff® G800110 - Power Brake Booster and Master Cylinder Assembly
 
Valkman said:
I thought the small reservoir(front) was for he disc brakes. I'll have to check out to see how I actually have
Click to expand...
no no no, the small reservoir is the DRUM! 4 piston caliper KH brakes X 2 requires some volume...besides your brake lines should not have interchanged due to different (stock) thread sizes.
 
OK, if you want a softer pushing pedal, and more brake control:

Put on a 15/16" bore manual brake master cylinder for the disc/drum application for the A Bodies.

Raybestos part # MC36406
Manual disc/drum master cylinder
15/16" bore

20200301_214423.jpg


20200301_214456.jpg


The stock size master cylinder bore for the manual disc/drum brakes are 1.032".

The 15/16" bore masters give you more leverage at the pedal with less foot pressure, and slightly more throw of the foot pedal, giving you more gradual control of your manual brakes.

People have said this gives the manual brake cars more of a power brake feel at the pedal. VS the hard pedal short throw, larger bore masters that come from the factory on the cars with manual brakes.
 
Valkman said:
Hey all, tried the search thing and there's just too much to go through. I currently have a 73 manual disc setup on my 65 Barracuda. The car stops great but it has a rock hard petal and I'd like to soften it up a bit. I have 2 questions:
1st is there a difference in master cylinder force (piston Dia.) and can I tell what I have.
2nd what booster would work and what (if any) modifications would be needed to add one?
Click to expand...
Yes with man brakes the peddle will feel hard and maybe high. If u have a cast iron master cylinder get rid of it! Go to a [salvage yard] an take.. booster, booster-bracket peddle-rod under dash, hard-brake lines coming off the master an propulsion valve,from any A Body. The master cylinder u want to use is the ones that are aluminum with the plastic reservoir. U can get them from any late Dodge van or FMJ Body's. Aspen Volar'e or local parts store. with a core. Last, adapter plate. The old cast iron master has 4 mounting holes, the new style master has 2 holes. the plate lets u bolt the 2 hole aluminum master to the 4-bolt booster. I did this to my 75 Scamp. Don't for get to bench bleed the new master. NOTE: You can not use the very large diameter boosters, they will hit your valve cover!
 
Valkman said:
1st is there a difference in master cylinder force (piston Dia.)
Click to expand...
yes; the bores vary from 7/8 to about 33/32 .. Each bigger size offers less foot travel, but has the potential to transmit more hydraulic pressure to the calipers, but will require more leg force.
Since disc brakes are NOT self energizing, all other things being equal, it requires more leg power to make the same-as-drum-brake stops. This is why you see boosters on almost all Mopar db cars.
Because the booster is so powerful, it is able to operate the big-bore MCs with very little leg-power. Now, if you transplant that big-bore MC, leaving the booster behind, then you end up with your situation.
So; you have a choice; re-install the booster, or get a MC with a smaller bore.
With the latter, there are a couple of things you have to work out.
1) Because the the new bore size is smaller, there is less volume of fluid in the chamber. So, it takes more foot stroke to push the fluid out; You better not run outta room to the floorboards, with a max-effort stop. But more importantly, if the rear system fails by catastrophic leak, your pedal will drop another inch to inch point five, during the brake application. Again, you better not run outta room.
2) because the MC bore is smaller, there is less fluid in the chamber. If your particular calipers retract too far from the discs in normal application, it becomes possible for all the fluid in the chamber to not be enough to push the pistons out AND still have full braking power.
3) this is why, for you, I recommended to find a booster, because your system is already designed for it. Just swap it in, set the pushrod length, remount the MC and yur done.
4) because db systems are NOT self energizing, this can be a problem for a non-boosted car, when stopped on a hill with the front of the car higher; the car will want to roll backwards. So you may have to apply more pressure. During a long traffic light, it might happen that you lose concentration, and the car rolls back into the vehicle behind you, and guess who's at fault. Same goes for if, for whatever reason, your car is on a fast-idle, in gear. A large-bore MC,without a booster, just makes all of this just so much worse.
So, when the question comes up; does your lightweight-A need a booster? Most of us might say no it doesn't. Your braking system can be engineered to operate satisfactorily, for a high percentile of persons, for a high percentage of the time. But I submit, that it's nice to have.
Valkman said:
and can I tell what I have.
only by pulling it off the firewall and measuring it.
2nd what booster would work?
any with same mounting pattern
and what (if any) modifications would be needed to add one?
adjust the pushrod length
Click to expand...
 
Last edited:
Thanks everyone! Great info. As far as the front and rear reservoir are on my car they're right, my brain was thinking backwards :(.
 
Thought I'd post some pictures of the difference in master cylinder bores
20200425_114337_resized.jpg
20200425_114831_resized-1.jpg
20200425_114820_resized.jpg


My search to fine a small diameter the small diameter one was not was not easy they're hard to find!
 
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