67 dart front disc upgrade made..still not right

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67dartgtgo

67dartgtgo

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Hi gang, have a 67 dart gt original 273 car with 360/300 in it. Here's the story. Piece mealed parts in stages due to bugdeting, lol. Originally had 4 wheel drum manuals. First change was power booster and new master cylinder, 7" dual diphragm 1 1/8 bore, while keeping the drums all around. Braking improved but still needed a very heavy foot to come to a complete stop.
Secondly wanted to keep small ball joint upper CA's and convert to front discs with large bolt pattern, so purchased an 11" disc brake at the wheels kit from SSBC. It installed fairly easily even though the instructions weren't the best. Installed an adjustable proportioning valve downstream of the metering block in the single rear brake line. Adjusted many times with no change. Bleeding was difficult as well with an extremely hard pedal while bleeding the rears.
The problem: Holy cow, fronts do grab some but braking is pretty terrible. Peddle is very hard but will stop the car with some needed distance. Definitely not drivable as is.

The breakdown:
21" of vacuum at the booster.
1 1/8" bore power MC from NELCOs Hotrods.
SSBC 11" rotors.
SSBC 4 piston calipers.
Aftermarket adjustable proportioning valve.

Bled brakes the traditional way, starting with the furthest away rear.
Any thoughts would be great.
 
So, the pedal is rock hard, the front brakes grab on fast and you don't have much pedal travel right?

I'd take a look at the master cylinder. 1 1/8" is BIG. 15/16" or 1 1/32" was used stock, and before you say that 3/32" isn't very big it makes a big difference when you talk about master cylinders. Sounds to me like you're pushing too much fluid.

Other possibility is the master cylinder push rod. What are you running for this? Power and manual brake set ups used different length push rods, and your master cylinder may use something entirely different.
 
72blunblu, pedal is rock hard but fronts grab just a little, not real braking at its best. Def won't satisfy that stopping feeling.
Really not sure about rod length but I did have to cut down the booster threaded rod in order to get it to fit on the firewall. Still have adjustment on that rod but not sure on the MC rod length.
Could it be the MC bore is too large a diameter. NELCO says that size is good for drum or disc power.
 
I would have to study OEM brake hydrolics to determine a correct answer but...
I think a proper perportion valve replaces a metering block that is found in 4 drum systems.
 
I would look into the master cylinder and booster rod lengths. Make sure you're getting full travel on the m/c, and that you're not bottoming out the m/c before you're out of pedal travel. The m/c piston has to be fully released when the brakes are off, and should be close to bottomed out when the pedal's on the floor, but you don't want the piston to bottom before the pedal.
 
Did you replace your manual brake pushrod with the bell crank setup? Did you push the brake pedal more than halfway to the floor during bleeding? Might be able to help after you answer those questions.
 
Get a shop manual, anywhere from 67--72 and read it

You can download a 66 and 70 manual from MyMopar

http://www.mymopar.com/index.php?pid=31

link

http://www.mymopar.com/downloads/servicemanuals/1970_Plymouth_Service_Manual.zip

You can download some manuals from a thread here, but some of the links are broken. Unfortunately, I think the 72 manual link is broken now as well PM AbodyJoe and see if you can get it. Some of the earlier links posted that are broken were reposted later

http://www.forabodiesonly.com/mopar/showthread.php?t=132309&highlight=manual%2C+download

Drum brake cars DO NOT HAVE a metering valve or prop valve. All they have is a combo distribution block, which also contains the piston operated warning switch. It might be possible that thing is all gummed up inside, has shuttled off to one side, and is plugging one port

You probably need to buy some fittings and a hi pressure gauge so you can check line pressures.

A "rock hard" pedal with a booster CAN mean a bad booster. Make absolutely sure the pushrod/ cylinder have enough room to return properly
 
Grump...all of the above are new.
Did some troubleshooting this afternoon...booster is good....push rod returning all the way.....shimmed out the booster further from the firewall to ensure rod length is sufficient, resulted in more pedal length but same pedal result.
67dart273...thanks so much for the links...those are great....i misspoke earlier when I said metering block, meant distribution block.
Thought about the block switch moving...tapped and brake pedal bled both front calipers at the same time...(2 man job)..no change.
Concluded MC bore is too large unless you guys have anything else you might consider or confirm :coffee2:
 
Does your master cylinder setup look like this....
 

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All signs point to bad booster, new or not.

But why are you running a booster anyway? Are you a polio survivor or something? Pull that booster and give it to some illegal immigrant kid to take home to his family.
 
Grump....booster and MC are aftermarket MBM combined unit...believe its part #BCK 5001...but I think I like yours better.

Jos...you are a very funny man...lol....live in the country a bit so no immigrants but my short leg gets a little weak sometimes...lol.

Would really just like to have real power brakes...I think....not totally convinced yet though.

Thinking I may start looking for a used unit (booster and master) soon.
 
Reason I posted the pic is so you can see what a Mopar power brake setup should look like. Notice the bell crank setup at the right. That is what MaMopar used to hook the brake pedal to the booster. By design it gives you an increase in mechanical advantage over a straight pushrod, meaning you don't need to press as hard on the pedal. If you don't have that, your power booster is not going to be as easy as you want. Before you buy another power set up, try a master cylinder from a '74 Charger with manual disc brakes. The last one I bought was 25 bucks at the corner parts store. It will bolt to the firewall without any mods and use your existing stock push rod. That old Charger had the best feeling brakes I have ever put a foot to and has been on every Mopar I have owned since, including a couple of A bodies. I have no problems with it and I have an artificial knee.
 
grumpuscreature said:
That is what MaMopar used to hook the brake pedal to the booster. By design it gives you an increase in mechanical advantage over a straight pushrod, meaning you don't need to press as hard on the pedal. If you don't have that, your power booster is not going to be as easy as you want.
Click to expand...
Backwards description. That lever applies less force on the MC (and more travel). That is why the factory MC on power brake cars has a smaller diameter bore than a manual MC. Some people think that is backwards, but not when you understand the lever arrangement. Lookup (rockauto) the factory MC size for power brakes for your year to get an idea. I would think a 7/8" MC is close, which would be 40% less pedal effort than your 1-1/8" bore (can anyone else calculate that?). I put a booster and MC from a Breeze on my Dart (7/8" MC, I recall). I still have the front drums. The brakes are a little too touchy now, but should be perfect when I change to front disks. For now, I have to learn not to press too hard and skid. If still no luck, take the booster off and move the rod by hand while connected to engine vacuum and see if it follows your motion without much resistance.
 
The drum brake and disc brake masters are different. Which one are you using?
 
Bill and Mike, I'm using the 1 1/8" bore MC, although the info I'm getting on other related sites is conflicting as well as some suppliers websites. I'm going to go with the 1" bore MC. It's $50 but if it does the trick I'm all for it.

I did remove the booster and ran the rod in and out under vacuum and it followed but not quite as easily as i would have thought but having never done it before I'm not exactly sure how much resistance should be there. Also tried to run it in and out with no vacuum and it def won't move that way. Concluded booster is good.
 
I have a 1968 barracuda with factory disc / drum brakes which were manual from the factory and I have added a power booster so my wife could more easily stop the car using the same stock master cylinder.

The master cylinder is different for drum / drum and disc / drum set ups, you do not indicate a change of master cylinder with the change to disc / drum. You said that you got the big 1-1/8" master when you still had drums all around?

The 1-1/8" master sounds like it is huge and pushing much too much fluid resulting in a much harder pedal.

The correct power set up looks like the one shown in another reply.

I would suggest getting a stock Disc / Drum master from your local parts house with standard push rod.
 
billd and all, installed 1" bore MC....lo and behold all is well....what a difference the bore size made. Car stops very well...no pulls and is easy on the leg. I wonder what it would be like with an 8" dual diaphragm booster.

Thanks guys.
 
Came in here late & saw that problem is fixed. Lager M/C bore= harder pedal effort to move the smaller brake pistons & therefore actually less braking power. You're moving more fluid at a faster rate per inch w/ the larger m/c into a small area(caliper, wheel cylinder). The brakes apply quickly but you cant move fluid faster into the smaller cavity(caliper, w.c.) than it can handle so you have increased pressure, but not increased travel on the working end. So less braking force. Think of it like hydraulic gear ratio.
 
A quick update....found an 8" booster cheap and installed. This is exactly the feel i was looking for along with great breaking.

340sFastback, I read your brake page on the 'cuda. Liked lots and super informative. Thank you.
Lonewolf...thanks for the breakdown on MC logic...surley makes sense.

Breakdown: 1"MC, 8" Booster, combination valve and adjustable proportioning valve.
Thanks again to all.
 
grumpuscreature said:
Reason I posted the pic is so you can see what a Mopar power brake setup should look like. Notice the bell crank setup at the right. That is what MaMopar used to hook the brake pedal to the booster. By design it gives you an increase in mechanical advantage over a straight pushrod, meaning you don't need to press as hard on the pedal. If you don't have that, your power booster is not going to be as easy as you want. Before you buy another power set up, try a master cylinder from a '74 Charger with manual disc brakes. The last one I bought was 25 bucks at the corner parts store. It will bolt to the firewall without any mods and use your existing stock push rod. That old Charger had the best feeling brakes I have ever put a foot to and has been on every Mopar I have owned since, including a couple of A bodies. I have no problems with it and I have an artificial knee.
Click to expand...

Will the master cylinder from a '74 Charger work on a '68 Dart with front disk/back drum?
 
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