Power Disc Brakes not working on hard stop?

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Dartman61

Dartman61

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Hello, I wanted to see if you guys had any insight into my power disc brake operation. I've got a 67 Dart GT 440, 4spd with power disc front brakes and rear drums - stock components. I have a manual proportioning valve to limit the rear brakes. This setup is 10 + years old and under normal driving conditions works fine. However, when applying brakes in a hard stop it feels like the pedal hits a hard stop but the brakes don't feel fully engaged. No matter how hard I press I don't get anymore braking power. I'm wondering if this is a limit of the early stock disc brakes or if I have something else going on? It's been like this since I put the car together and I'm finally going through the car and tweaking the stuff I'd like to work better. The question is this normal and if not what can I try to correct?

Thanks in advance
 
How feels the pedal? Spongy to the floor or rock hard with a 1/2 inch travel?
Hydraulic ratio could be a issue, a too big mc creates a hard pedal.
Glaced brake pads and disc brakes. Whats your setup?
 
Pedal is rock hard, travel is "normal" 1 inch or so. I believe the MC is 1 inch bore. It's stock MOPAR stuff. I'll take a pic of MC and Booster
 
A) How much vacuum is the engine producing?
B)Engine off, pump the brake pedal 4-5 times.. Now, foot on the pedal, start the engine.. Does the pedal drop slightly?
 
So you can't get the tires to skid?
 
Vacuum is low but adequate and power portion of brakes work great. Pedal drops on start up etc. No I can't get the brakes to lock up.
 
Dartman61 said:
Vacuum is low but adequate and power portion of brakes work great. Pedal drops on start up etc. No I can't get the brakes to lock up.
Click to expand...
When was the last time you bled the brakes?
 
Power brake boosters have an adjustable plunger to the MC if it is set with too much gap you will have to little pedal travel to fully move the mc pistons.

( too little can move the mc piston in a bit and cause it to block ports that are needed for normal operation)

Someone else made the point of the mc bore diameter. Smaller diameter means less leg pressure but more pedal travel, larger diameter is less pedal travel but more leg effort.

With power brakes you don't need less leg effort so you should have the OEM size or larger.
 
OK a bit more about the system - picture attached. The calipers are 4 piston and along with rotors are a stock setup correct for a 1967 A body. The MC and booster I believe are also a stock setup, 1967 was first year for dual lines on MC for front/back. There is no air in system as I use a motive auto bleeder kit from Summit ( well worth the $60 - I highly recommend - Motive Products 0100 Motive Products Power Bleeder Kits | Summit Racing). I recall something about the rod adjustment that Dana 67 Dart mentions and can revisit that. Does anyone have good instructions for doing the adjustment? I will Google but that normally lead me back to this site :) Thanks for the good feedback.

67 Dart MC Booster.jpg
 
This sounds like a booster problem. The booster has internal valving to modulate the line/output pressure, which I suspect is faulty. Low pressure on pedal=low output pressure; higher pedal pressure= higher pressure.
 
a regular oil hose to the vacuum canister can collapse, so there is no vacuum at the booster! they are build for presure not vacum
 
I don't think its a vacuum or canister issue. The pedal operates freely and power assist feels right. It's just on a hard stop I seem to run out of pedal which limits the pressure to the brake. I'm hopeful adjusting push rod will help. Will advise when I've been able to adjust and test.
 
Dartman61 said:
Tube is capped - hard to see but there is a blue cap on the valve
Click to expand...
Might want to run your fingers around it with the engine running... A minor leak there will cause a hard pedal DAMHIK... Had a vacuum cap that got a hole sucked through..
 
Mike69cuda said:
My car has the SSBC power disc brake kit
Click to expand...
What kit did you get - I just put one in a 67 RT - same issue. The kit I got was the A156-1 which came with the MC & Booster but very little if any direction. Had a faulty caliper which they replaced. I'm hoping this pin adjustment will work and it not just a limit of the system.
 
My brakes work fine until I brake really hard. The pedal feels like it has a board under it and it is not near the floor. A very hard stop, the pedal won’t go any farther. Sounds very similar to the OPs.
 
Mike69cuda said:
My brakes work fine until I brake really hard. The pedal feels like it has a board under it and it is not near the floor. A very hard stop, the pedal won’t go any farther. Sounds very similar to the OPs.
Click to expand...
YUP that is my experience exactly with both the stock setup and the SSBC - I'm hoping adjusting the pushrod will eliminate
 
Post #8, imho
Most times a customer complained of similar, rear brakes were so loose, the whole master cylinder stroke goes to moving the shoes out to contact the drum. There can be no "pedal" till all pads/shoes are in contact with rotor/drum.
The m/c runs outta travel. (I know you think it shouldn't)
Pedal goes "bump"
Nothing has changed in 10 yrs, why would m/c pushrod change, plumbing didn't change, fluid levels up, bled.
Shoe wear is easiest to eliminate, piece of shoe coulda come off.
.jmo
Good luck
 
Sorry that makes no sense. MC is 2 separate chambers the only way for front brakes to be impacted by the rear is for the rear to limit pedal travel. In your scenario with rear break wear that would just allow more pedal travel and put the pedal closer to the floor which is not happening. If anything with more travel required by the rear brake it should put more pressure on the front brakes. Sorry but when its not right its just not right
 
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