Power Disc Brakes not working on hard stop?
Thank you everyone! After reviewing all the responses and evaluating my experiences with 2 systems (SSBC conversion on a 67 RT Coronet (SSBC uses the same setup on B & A bodies) and my 67 Dart (Stock MOPAR setup), I believe I'm experiencing the natural limitations of the smaller rotor and disc brake system. Simply put, its at the end of its braking power. The only way to overcome it is to go to larger calipers and rotors, which would require larger, bolt pattern with larger wheels and tires. I've traded emails with both Dr Diff and SSBC and neither can offer any cures with my current setup. Steps I've taken: Power bled the system, measured the MC pushrod to ensure max fit and travel, checked all rubber lines, checked for vacuum leaks and found none, ensured booster and MC operate as expected. The brakes work great under normal circumstances with good stopping power and easy pedal. When I brake aggressively while going fast I reach a limit where more pressure doesn't provide more braking ( a hard pedal not on the floor). The fact that I have the luxury of testing 2 different cars with the smaller calipers and rotors and they act similarly leads me to believe this is normal. While I was hoping to find a way to improve I discovered this is the best 1960's technology had to offer. I appreciate everyone's contributions.
NO NO NO NO.
I have the KH calipers, and have tried every Mopar M/C from 7/8 to 1, and every one of them has worked perfectly. I liked the 15/16 best and so that was where I settled.
I'll give you the short answer first;
This same thing can happen anytime the PowerPiston hits the end of the chamber.
Such as,
1) like you suspect; the M/C runs out of stroke-capacity or
2) the C-ports are obstructed, or
3) you got too much piston knock-back, or
4) the hoses have weak walls, or
5) the discs have too much runout, etcetera,
the point being that for some unknown reason, the caliper-pistons have been forced too far back into their bores. With FOUR of them, this is easily possible.
You can prove this.
During braking just pump the pedal in rapid succession before slamming the pedal down hard. The pedal must return to the top after each stroke to open the C-port, but you gotta get that done before the fluid has a chance to return to the reservoir. If you no longer "hit the board", Badaboom.
that was the short answer.
Now a short explanation;
If you try this with the vehicle running but stopped, you can eliminate knock-back from rotor runout and from loose wheel-bearings.
Once you have ascertained that this is the problem, now you gotta figure out how to cure it.
And now the long rigmarole that Rumble hates;
In the Meantime, I have to ask; is the M/C is plumbed correctly from the M/C to the Combination valve and out to the Calipers. You know that among all the permutations that this can be done in, there is only one right way, .... right?
Ok to be sure;
No matter what type of Valve you have; be it a
Combination valve, or a Proportioning Valve, or a Metering Valve, or even a Y-block, or even if you have plumbed in a manual rear proportioning valve in conjunction with some other valve;
the rearmost chamber of the M/C always goes to the front brakes, with no proportioning of any kind.
Now;
inside the M/C, there is an assembly of two power-pistons mounted on a shaft, one piston for each end of the car. The rearmost is fixed to the shaft, while the front one is free to slide back and forth, located only by the spring between them, when the thing is empty. After filling and bleeding it, That entire chamber is filled with brake fluid, and the frontmost piston will be hydraulically coupled to the rearmost, and they will move together as a unit. Well they're supposed to ...
When you step on the pedal, the whole thing moves forward as a unit, pushing fluid towards the slaves, which are the calipers in the front and the w/c's in the rear. If for some reason, one end of the car has failed, then the whole thing collapses, and is driven mechanically by a stump on one or both of the Power pistons.
Lets say the rear has failed. which is connected to the frontmost chamber. Then both pistons will slide forward, until the frontmost piston hits the limit of it's travel, and then fluid can begin to be pushed out to the calipers. You would feel this in the cab, as additional pedal travel.
Lets say the front has failed. which is connected to the rearmost chamber. Then the hydraulic inter-piston chamber will still drive the frontmost piston in a normal fashion, you just have lousy stopping power, and additional pedal travel. If you had a working Combination Valve, it would turn on the dashlight and inform you of the failure.
Now lets suppose that the hydraulics are working as intended.
You say that you cannot lock the front wheels.
Now you have to figure out if this is a hydraulic problem, or a mechanical problem.
Since you have a high and hard pedal at the beginning of the stroke, AND you don't mention an extended pedal travel, I'm gonna assume the hydraulics are fine; which leaves just a mechanical issue.
You can prove this by removing the calipers and all of the rear brake bits; then installing C-clamps to retain the Pistons and more C-clamps to secure the rear pistons, THEN, stepping on the pedal. If you did not have a booster, the pedal should not budge. But with a booster, there will be some travel. then it locks up high and hard.
But I think we can dispense with that.
That leaves a mechanical issue. In your case I can't think of one unless the pads hang up, which I've never seen these 4-pistons do, or unless the hoses have an intermittent obstruction in them, which I have seen, but never both at the same time..
So let's go back to the master.
In the bottom of the reservoirs are several holes. the bigger ones supply fluid to the power pistons. The smaller ones are for compensating for additional fluid as the pads and shoes wear, and to fill the inter-chamber. In order for these to work, the pistons have to return to their parking spot at the beginning of the bore. and in order for that to happen, the booster has to pull the pushrod back, which then parks your brake pedal up where it belongs. You hear the little thunk when that happens, after every pedal up-stroke, when you take your foot off the pedal. You can tell when the C-ports have quit working because the pedal sinks ever lower over time, and then one day, there is just not enough pedal travel to stop the car. with, eventually, a "hitting the board",
But what if, two things go wrong
1) C-port is not open, and
2) the booster is not properly connected to the pushrod in the M/C.
In this case, the pedal parks just as it always has, but the Power-pistons are running out of travel. So one day you step on the pedal, and the PowerPistons go out, but slam into the end of the M/C before having energized the slaves, or in your case only the fronts may be involved. In this case you have no pedal resistance at the top of the pedal travel, but everything seems normal after that, until you need that last bit of travel in the front system, on a hard stop. When the PowerPiston assembly hits the limit of travel, you feel it like it's "hitting a board," as the hydraulics lock up.
Is that the only scenario?
No. This same thing can happen anytime the PowerPiston hits the end of the chamber.
Such as,
1) like you suspect; the M/C runs out of stroke-capacity or
2) the C-ports are obstructed, or
3) you got too much piston knock back, or
4) the hoses have weak walls, or
5) the discs have too much runout, etcetera,
the point being that for some unknown reason, the caliper-pistons have been forced too far back into their bores. With FOUR of them per side, this is easily possible.
You can prove this.
Just pump the pedal in rapid succession before slamming the pedal down hard. The pedal must return to the top after each stroke to open the C-port, but you gotta get that done before the fluid has a chance to return to the reservoir. If you no longer "hit the board", Badaboom.
If you try this with the vehicle running but stopped, you can eliminate knock-back from rotor runout and from loose wheel-bearings.
Once you have ascertained that this is the problem, now you gotta figure out how to cure it.
For instance, I have the same 4-piston KH set-up on my Barracuda; I took it off a 70 Duster, all of it except the Master, and my booster ended up being from an F-body. I like a lil knock-back cuz my car takes a like a mile to coast down from 65 mph, in neutral, meaning that my brakes are not dragging my fuel-mileage down.
There is a monkey-wrench in this system. It assumes the brake fluid is always a liquid. If at any time it boils, or if it is contaminated with water and the water boils, now you have a gas in the system, which is compressible, and down goes the pedal, with insufficient ability to stop, and bam, you "hit the board". The same thing can happen if the inter-chamber between the PowerPistons, has air in it, but with one difference; in the event that air is in there, you will get a spongy pedal, which you didn't mention.
Ok that's all I got Best of luck to you.
Ok I thought of another thing.
What if the Frontmost PowerPiston was installed backwards, I don't think this is possible, but if it was, then the stump on it would be facing the wrong way and would become a travel-limiter.
