Soft brakes on 4-wheel disc conversion

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Duster360LA

Duster360LA

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When I bought my 74 Duster project, one of the few items on the car were Year One front disc brakes (which were installed on the wrong sides so that bleeder sales were facing down). It also came with a disc brake conversion kit to go on the Dana 60. It came with a new power brake unit (uninstalled) and I replaced all lines and the distribution block.
Fast forward and I have wrapped up the build. Car runs great, but brakes are an issue. The pedal travels down about 1.5 to 2 inches and stops no matter how much foot pressure you apply - and you slowly coast to a stop.
I put an adjustable proportioning valve in the rear brake line, but small adjustments to it don't have any impact on braking power. No leaks and have bled each caliper multiple times.
Any suggestions?
 

Should have mentioned that I hooked up the emergency brake and it works great.
 
Four things.

  1. Bleed bleed bleed
  2. Bleed some more
  3. Some rear disks have or need initial adjustment. Check with the manufacturer and find out what make and model they are from.
  4. Be sure the push rods are adjusted correctly. And the correct ones for the application
 
Are you still running the booster or just M/C? What size bore is you M/C?

and as others will tell you bleed from the furthest point, usually the right back and work your way forward to the shortest point!
 
Duster360LA said:
The pedal travels down about 1.5 to 2 inches and stops no matter how much foot pressure you apply - and you slowly coast to a stop.
Click to expand...
have you checked the pushrod lengths-- pedal to bell crank, bell crank into booster, booster to master.

if i was a bettin' man, i'd guess that the booster to master needs an adjustment.
 
Running the booster on the MC. No literature on the MC, but will see if I can find a part number and get a bore size.

Engine is a 360 with a mild street cam. Sniper 2 EFI. I'll have to check vacuum.

And will continue to bleed system and check pushrods.

Thanks

20251112_093654.jpg
 
I ran into the same problem with power brakes and disk set up on the front and rear. I ended up scrapping the brake booster and went back to manual brake master cylinder for four wheel m minddd disk and a adjustable proportioning.
I think my problem my have been my “pedal ratio”
 
Sounds to me like the problem is likely the total travel of the petal, pushrod, and linkage ratios which are off.

Lots of people install the calipers with the bleeder in the wrong direction, it’s okay if you actually bleed it off the car with like a 2x4 in between the pads with the bleeder up and then re-install the caliper afterwards. Not I deal, but possible.

Did you bleed the master separately?
 
Confirm you used the 4 wheel drum brake distribution block and not the front disk/rear drum distribution-metering block?

There's a small, inexpensive tool available that measures the clearance for adjusting the pushrod into the master cylinder. Get one of those and use it.

As mentioned above, make sure the bleeders are at the very top of the calipers when bleeding. I had to remove mine and block the pistons from popping out of the caliper.
 
jbc426 said:
There's a small, inexpensive tool available that measures the clearance for adjusting the pushrod into the master cylinder. Get one of those and use it.
Click to expand...

www.summitracing.com

Wilwood Disc Brakes 260-18027 Wilwood Master Cylinder Pushrod Length Gauges | Summit Racing

Free Shipping - Wilwood Master Cylinder Pushrod Length Gauges with qualifying orders of $109. Shop Master Cylinder Tools at Summit Racing.
www.summitracing.com www.summitracing.com

jbc426 said:
I had to remove mine and block the pistons from popping out of the caliper.
Click to expand...

+10000 to removing the rear calipers and blocking the pistons to bleed off the car. especially with those gawd forsaken GM metric calipers and the "rat-****" adjusters.
 
Thanks everyone! I'm starting with the pushrod since that seems to be the most popular culprit and work my way through the other suggestions.

And yes, the MC was bled separately.
 
looks like a disk/drum m/c on a 4 wheel disk car
maybe you need a larger size m/c might need more volume
does the peddle pump up or is it just low
did you just add rear disk to the system you had
 
The pedal is firm and only goes down about 2 inches - the problem is that I only get marginal brake power when the pedal has significant pressure on it. When I bleed the system, the pedal goes to the floor and doesn't need much pumping up.
The piston rod length that several members brought up was an issue. Using the tool they suggested, the rod was about 1/4 inch too long, so I adjusted it. I'll take it for a road test tomorrow.
In terms of the braking system, most of the components came boxed up when I bought the car. New power booster and MC, new front and rear disc conversion (Year One). I bought all new lines and a distribution block. Based on the fender tag, the car was originally manual brakes.
 
Duster360LA said:
The pedal is firm and only goes down about 2 inches - the problem is that I only get marginal brake power when the pedal has significant pressure on it. When I bleed the system, the pedal goes to the floor and doesn't need much pumping up.
The piston rod length that several members brought up was an issue. Using the tool they suggested, the rod was about 1/4 inch too long, so I adjusted it. I'll take it for a road test tomorrow.
In terms of the braking system, most of the components came boxed up when I bought the car. New power booster and MC, new front and rear disc conversion (Year One). I bought all new lines and a distribution block. Based on the fender tag, the car was originally manual brakes.
Click to expand...
Yah, the Disc/Drum Metering Block won't work with 4 wheel disc, as you know.

I am using the over 1 inch bore MC on my power brake booster car (E-Body w/4-wheel disc), and it works nicely. I did have to go with the just under 1" bore on my manual brake set-up (A-Body w/ 4-wheel disc) to get the right feel and not have to push so hard on the pedal to stop.

I also added this rear brake "anti-lock" device on my E-Body, as it has always tended to lock up the rears even with drums. It made a huge difference, and is relatively cheap and easy to install in the rear line just before the flex line to the rear end.

Here's a picture of it ad a link for those interested.

www.larsenracingproducts.com

Lock Resistant Brake System (LBS)

Ensure maximum safety and control with the Lock Resistant Brake System (LBS). Engineered to prevent wheel lockup, this advanced braking system provides superior stopping power and stability, ideal for high-performance and everyday driving conditions.
www.larsenracingproducts.com www.larsenracingproducts.com

Brake anti-lock 1.jpg


Brake anti-lock 2.jpg
 
Never heard of it
How does it work
Funny now that you posted
For those who like to play
A lot of cars have load sensing rear proportioning valves
Might be a nice trick set up
So when the back lifts under heavy braking it releases pressure to the rear
 
nutz said:
Never heard of it
How does it work
Funny now that you posted
For those who like to play
A lot of cars have load sensing rear proportioning valves
Might be a nice trick set up
So when the back lifts under heavy braking it releases pressure to the rear
Click to expand...
No, it just cushions the initial shock wave to the rears brakes, but does not reduce the line pressure otherwise. The web page goes over the function of it nicely.

It is not a proportioning valve, which can be nice when track/driving conditions change; but is often used as a bandaid for improperly matched front and rear brake bias. Matching front and rear clamping force by properly sizing brake components is where setting up a brake system should start.

Brake part.JPG
 
I wasn't able to get a solid pedal until I rotated the rear caliper pistons all the way in, used emergency brake to push them all the way out, then rebled brakes. Mine is the Dr Diff 10.7 kit with the mustang cobra calipers.
 
I did put a proportioning valve on the rear line and continue to adjust it to see if it makes a significant difference. May call Dr. Diff about a larger bore MC.
 
Circling back on this issue. Given the front disc's and calipers were installed 5+ years ago by whomever started work on this car, we decided to tear down the front calipers so we could clean them and put in new seals (and found out that needed to be done). We re-bled every wheel and the pedal has a much better feed to it, but still work exert enough pressure to lock the wheels up on a hard stop.
We then took another manifold vacuum test and drew 15 pounds at idle, which seems like it wouldn't be enough to support the power booster. The MC and booster were purchased 5+ years ago with the front brakes. Lines, distribution block and proportioning valve I purchased less than a year ago.
 
Duster360LA said:
I did put a proportioning valve on the rear line and continue to adjust it to see if it makes a significant difference. May call Dr. Diff about a larger bore MC.
Click to expand...
Not sure if this needs to be said, but that the larger the bore size, the more the pedal pressure will be required to produce the same line pressure.
 
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