Help me make my brakes suck less

Now that I've finally got my brake bias set up just under not locking the rears, I'd like to solve my longstanding why-aren't-these-that-good disc brake problem. It's on an old Mopar so I've wasted a lot of time getting nonsense and unwanted recommendations from Mopar forums :P

Car: 1967 Dodge Coronet station wagon, about 4000 pounds o butter with half a tank and me in it.

Original Setup: 11x3 inch drums front

11x2.5 drums rear (15/16" WCs)

Manual 1" MC

Current setup: 11.75 inch discs front (Cordoba) with Wilwood 2.75" GM calipers (using this kit)

11x2.5 drums rear

Manual 1 1/32" MC

Yes it has a prop valve

Despite bleeding the crap out of all ends and even throwing one of those little weenie check valves on the rear line, the pedal always has some slack in it. I can lock up the rears if I have them adjusted out really close and the prop valve too far in, but I cannot lock up the fronts. It stops well enough, but if I put my foot in it as hard as I can it will not lock the fronts much less stop as hard as it seems like it should for the components.

My best guess at this point (after obsessively bleeding everything and accepting my rear drums are always going to make it squishy-ish) is trying a smaller 15/16" MC, but I'm worried it will just give me even more pedal travel without more force up front. Am I missing anything?

What kinda pads? My Celica wouldn't lock at all until I got rid of the autozone pads for some EBC Reds. And even then, I had to heat the pads up to get them to do a nice, flats potting lock up.

I'm using Wilwood's "ProMatrix" pads for these things. It's supposed to be an aggressive pad and they're definitely bedded in.

If it isn't locking the fronts and the pedal won't pump up, you've probably still got air in there. How was the master bled prior to install?

You could try a reverse bleed, pushing fluid up though the caliper bleeders.

Secret_Chimp wrote: Despite bleeding the crap out of all ends and even throwing one of those little weenie check valves on the rear line, the pedal always has some slack in it. I can lock up the rears if I have them adjusted out really close and the prop valve too far in, but I cannot lock up the fronts. It stops well enough, but if I put my foot in it as hard as I can it will not lock the fronts much less stop as hard as it seems like it should for the components. My best guess at this point (after obsessively bleeding everything and accepting my rear drums are always going to make it squishy-ish) is trying a smaller 15/16" MC, but I'm worried it will just give me even more pedal travel without more force up front. Am I missing anything?

Have you tried putting both feet on the brake pedal, just to see if that will lock it up?

Are you running out of pedal travel? If not, then yes you can get more hydraulic advantage with the smaller diameter master. You won't change the amount of one without chaining the other, the hydraulics involved will have to trade them off.

So you converted from the fronts from drum to disc? AIUI, drums will "wedge" the shoe into the drum when you apply them, which is why you can get away with a non-boosted setup on a 4000 pound car. Disc brakes don't have that mechanical advantage, you need to apply all of the force hydraulically, which is why you don't find non-power brake cars over 2000 pounds.

IMHO you need to put in a brake booster.

Had the same problem with my duster.

Only cure I found was a brake booster. Tried everything short of that with no success.

And a mopar booster will run on 11 inches of manifold vacuum.

If the brakes are "good" with normal pressure and normal stops but noticeably lacking under harder stops, is it possible the flexible lines are ballooning under the increased pressure?

(I have a feeling you're using braided lines in the kit already though)

I'll throw my two cents in with a vote for a booster as well.

Also, we need pictures of this wagon of which you speak!

Few random brake thoughts.

Beware high spots in the lines. Air bubbles can and do hide there, refusing to come out through conventional methods. Sometimes using two people and cracking fittings gives wonderfull snap/crackle/pop sounds and a firmer pedal. Especially when pedal pressure is applied, then the fitting cracked.

Worn rotors and drums eat up a lot of pedal travel getting the pads to push down against the metal.

Sticky caliper pistons sure do cause long travel and frequently a squishy pedal.

Same can be said for worn/dirty calipers that don't move freely, or don't let the pads move freely.

Maladjusted drum brakes = long pedal travel.

Belled drums always give squishy pedal.

Soft flexible lines give squishy pedal without necessarily blowing up like a balloon. Sometimes they just sorta move instead. They shouldn't move when you apply the brakes.

Cheap brake pads/shoes can have remarkably squishy compounds.

OK, I agree with the booster theory as well.

However, GM calipers can be installed on either side. If they are on the wrong side the bleeders are on the bottom of the caliper and the air will never come out, thus a soft pedal. Bleeders must be on top. I have found this on several DIY cars/trucks brought in on Monday.

Brakes can get complicated in a hurry once you start mixing and matching parts. Not sure if you are working off a "Known recipe for brake conversion" or doing your own thing. I will toss this out there in case it applies.

One of the most common misconceptions is that a larger master cylinder will create more pressure. While a larger master cylinder creates a larger displacement, it takes more force to create the same pressure as a smaller bore. While a larger master cylinder will take up system slack with less pedal stroke, it will take more force to create the same system pressure. The result after adding the larger master cylinder is a harder pedal which needs much more pedal pressure to create the same amount of braking force. For instance, moving from a 3/4" master cylinder to a 1" requires 77.7% more force on the push rod.

So what I hear is" "If you want a short stroke pedal with a lot of force, you need a booster or a gym membership." NOW I see why even small cars have brake boosters.

What about inserting a brake pressure gauge to see what you are getting at the caliper or wheel cylinder like this?

link

Spitsix wrote: What about inserting a brake pressure gauge to see what you are getting at the caliper or wheel cylinder like this? link

Hey, that is cheating! But a great idea. Where do you tap into for readings? Bleeder screw?

I think you are supposed to have at least 1000 psi for disc brakes.

In reply to NOHOME:

This one has adapters for different bleeder screw sizes in the kit Link

My new outlook on life is try to work smarter - Only took me 57 years to learn that one that alot of other people already knew!

Dusterbd13 wrote: Had the same problem with my duster. Only cure I found was a brake booster. Tried everything short of that with no success. And a mopar booster will run on 11 inches of manifold vacuum.

What was involved for you for adding a booster? I considered it initially when I was piecing everything together but kept coming across answers like this: http://www.forbbodiesonly.com/moparforum/showthread.php?73015-Adding-brake-booster-to-67-Coronet-with-manual-drums

Did you need to swap out everything the folks there mention? I keep hoping I can just whack a booster onto the firewall but when I go to research I find this stuff. "You need parts, no you don't need parts, no not for that year but you gotta do this other thing maybe, I didn't do that thing I just did X and Y and tweaked Z..."

WRT to bleeding posts - Bleeding-wise, if there is still air anywhere in this system I'll eat my hat. I've pulled through at least two big tall bottles of brake fluid with a Mityvac.

I bench bled the MC, later bled it on the car by pumping it up and cracking the lines (you can reach under the hood opening from behind with your foot on the pedal - fun!), bled the calipers by undoing the top slider and flipping the whole caliper up to get the bleeder up high, hit the back of the piston bore with a mallet to knock loose any air bubbles, pulled at least a pint through both sides of the rear axle, blah blah blah.

i'm going to make some generalizations here for the sake of simplicity:

for a given rotor or drum diameter (which is primarily driven by the wheel diameter), a duo-servo drum brake will make approximately 3x the torque of a disc brake for a given input pressure.

that caliper you're using is taking a whole lot more fluid to build pressure than the wheel cylinder that it replaced.

larger diameter master takes more force to make same pressure as smaller master (but it does provide more fluid volume per unit of pedal travel).

so yeah, put me in the "you need a booster" camp. i would go to the junkyard and get a complete booster and master cylinder from an older Ram 1500 or Durango -- just to keep it mopar, you know. those donors are front/rear split, with front disc and rear drum.

When I did the duster, I grabbed a junkyard setup for an a body. Just for the linkages and brackets. But I wanted bolt in and factory appearing. If I hadn't been worried about that, the bolt pattern on the rear of an s10 blazer from the late 80s was awfully close. Then I would have had to adapt the booster/master combination to my pedals/lines/firewall. But the gm boosters work better.

If you'd bled it that much, I'll move to the "not enough pedal force/line pressure" camp. 4000lbs and manual brakes just don't mix well, worked OK with the drums because they self apply to some degree when they're properly set up and adjusted. Did Chrysler ever offer it with manual disc drum?

All you can really do is install a booster of some sort, vacuum or "hydro-boost". Altering the leverage in the pedal linkage might work, but leads to other issues (might run out of pedal travel under heavy braking).

Well it does sound like I need more than my feet and my seat. If there are any parts left in junkyards around the Seattle/Snohomish area that would fit my car, I don't know where they are. Everything is pretty picked-over.

Though this doesn't look too bad: http://www.ebay.com/itm/1962-74-Mopar-Dodge-B-Body-8-Dual-Power-Brake-Booster-Conversion-Kit-/161567942667?hash=item259e33280b&vxp=mtr

I never understood why boosters for these cars ranged between the sub-$200 range to $400 to $700+

seems pretty good. standard universal streetrod booster/c3 corvette master. brackets look adequate depending on what theyre made out of.

pull the trigger and get the wagon going. or stopping....