I have a 68 Barracuda, 367/4-speed/3.55s/ at 3650 pounds/me in it.
In the back are 295/50-15 on 10x2 drums, also no proportioning.
In the front are 235/60-14s on KH 4-pot calipers on something like 10.7 discs
The master is a 15/16 Disc/drum; On the back are 15/16 w/c's
the Booster is off a 73>75 Dart.
The front hoses are braided, the rear is a stocker, the hard lines are 1968,
and all tires are BFG-TA's, all pads and shoes are stock Mopar replacements.
This car for a streeter, stops amazing well, like I imagine throwing out a parachute at ludicrous-speed might feel like. I mean slam your face into the dash if I don't warn the passenger. Nobody wants to ride with me anymore so I took the rear seat right out. She easily locks the front brakes at 20 mph.
Soooooo about the only thing different is that your Duster is a lil lighter.
So what's different?
If your pedal is hard, high when braking, does not sink under your foot, and you cannot sense any spongyness, then the hydraulics are fine.
Your friction materials should be fine.
AFAI can tell, that leaves maybe two things.
1) are you braking with trans still in gear?
If yes, does your car nose-dive when you take your foot off the gas-pedal; which with a 440/manual trans, it should do. If your engine is slow to return to idle and you are trying to brake the car with the trans still in gear, then your brakes are gonna be trying to slow the engine down too! That's a bad situation. Yur not doing that, ........ are you?
I mean if your engine has 185psi CCP or more, like mine once did, I hardly even need the brakes, as just lifting off the throttle, slows the car fairly well.
2) mechanical issues. Like as if the pads are not dead-FLAT, or are not in full contact with the discs during braking, or not free to move in the brackets, or free to retract off the discs, or the Park-brake is not adjusted right, or the hoses are rotten and/or flexing, or like JYH suggests, maybe the lines are reversed off the M/C, altho, without any proportioning it might not make a difference.
NORMALLY,
the frontmost chamber, nearest the rad, is plumbed to the rear brakes.
If you are using a gutted factory MOPAR combination valve, make sure the shuttle valve is centered and the brake-warning lamp works, and is off.
Here's a simple test;
Find a hundred yards of slightly dirty concrete road with no traffic. Have a friend stand off to the side to watch your tires. Then, Run up to it at 10 mph and hit the brakes hard enough to get at least both of the front tires skidding. If your helper sees all tires skid, great! Hit it again at 15 then 20. By 20, the rear brakes should not be skidding any more.
If the rears always skid, regardless of what the front is doing, that's bad, and has to be changed.
If the fronts cannot be caused to skid, at all, that is also bad.
If no tires skid at all, holy crap! that's terrible.
In your case IDK which is worse, lol.
BTW, IMHO, an all-disc car should have a good working booster.
Here's a some clues and stuff.
Firstly; Inside the M/C are installed two power-pistons, with a hydraulic chamber between them . When you step on the pedal, you are moving the piston nearest the firewall. The Frontmost piston is Not physically connected to the rear one. Instead, if you bench-bled the M/C correctly, and it is full of fluid, then the frontmost piston will be forced to move ahead by the advancing rearmost piston, by hydraulic coupling.
Each of those pistons, is gonna drive fluid down the line just a short distance.
When the M/C is plumbed correctly, the Frontmost piston will be driving fluid into the rear circuit, while the rear-most piston just slides right on by, but not past, the port to the front caliper pistons. When the rear pads finally sit on the rear rotors, this stops the frontmost power piston from moving. The rearmost piston is now ready to move fluid into the front-brake circuit. As you continue pushing, the rearmost piston drives fluid into the front calipers and the pads move out to the rotors.
Until now no braking action has occurred. But now everything is read to go to work; and you feel it as the pedal goes hard, with about an inch of travel so far.
Continued force by your leg, now increases the fluid pressure, which squeezes the pads ever harder onto the rotors. Stopping follows.
That's how it is supposed to work.
>One of the things that sometimes causes problems is too much wheel-bearing clearance. If the rotors are allowed to flop around even a little, they will drive the pistons down into their bores, away from the rotors. So then on the next brake application, the Power-piston has to waste time and pedal travel, to push the caliper pistons back into position.
The caliper-Pistons are supposed to be retracted by the square-section O-rings, which get stretched a lil on every brake application. When you lift off the pedal, the O-rings are supposed to pull the pistons off the pads just a couple of thousands, so they don't drag. Eventually, as the pads wear, the O-rings slip, just a lil.
But, if the bearings are too tight, then possibly the O-rings cannot do the work. I run my bearings on the loose end of the scale so that "knock-back" assists with pulling the pads back. I never notice the additional pedal travel.
> With just a 15/16 M/C bore, there is not much fluid travel on pedal application; whereas with 4-pistons on each side, those calipers require quite a bit of charging-fluid, if the pistons are knocked back too far. But you will recognize this when it happens because the pedal-travel continually increases.
> Now, I want to mention one thing that happened to me on the job, a long long time ago, on an all-disc car.
I couldn't bleed the fronts. The rears bled fine and even worked! without the fronts. But I just couldn't get fluid down the front lines. In desperation, I opened the rear bleeders and down went the pedal. Ok fine, I bled the Fronts, then the Rears and all was well in the world. Car braked perfect so I sent it, and never saw it again.
I'll tell you the truth; To this day, IDK why this happened and in ten years of being a General-mechanic, at that shop, it never happened again.
Just maybe, you're number 2.
