Delayed Disc Brake Release

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Chuck Henderson

Chuck Henderson

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I’ll try to be brief…

1970 Dart, ABS Powerbrake (Orange, CA) front disc brake upgrade kit for the original 9” Drums, 5x4” pattern.

Issue; After braking to a stop, both front brakes are slow to release (3-5 seconds) after I release the pedal. If I jack up the front end and have my wife step on the brakes, I can’t turn the wheels for a few seconds.

Supporting info;
  • system normally installed and bled-I did a similar upgrade on my ‘69 Barracuda (SSBC)
  • I had to grind the stock spindles for caliper clearance, they slide freely
  • new hard and soft brake lines
  • new disc brake master cylinder without a residual pressure valve on the front discs
  • proper/stock brake cylinder push rod
  • no discoloration on the rotors or appearance of an overheat situation
  • This happens to both wheels
ABS is no help, really, and the only thing I can suspect is the distribution block As it’s the only thing I haven’t checked. (I swear I replaced it with a combination‘proportioning vale and distribution block)

ideas?

Thank you,
 
Sounds like the piston in the master cylinder isn’t fully returning to expose compensating ports. Is there anything not allowing the pedal to return fully, brake light switch? When it happens try looking at piston on master cylinder from under dash to see if it’s traveling back to retainer. Could just be a defective master cylinder also.
 
Thanks Dubois, I’ll check it.
I failed to mention that I replaced the MC recently as I thought perhaps it was the culprit and I wanted to replace the 1” bore MC with a 15/16” bore as they’re manual brakes.

also, do any A-Bodies have brake pedal return springs other than the spring(s) in the MC?
 
No spring. Just the one in master cylinder. I personally would move the brake light switch out of the way to see if it’s the issue.
 
I believe that is normal. Brake shoes have springs to return them but caliper pistons don't. Piston "return" is a function of piston design, rotor runout and lack of residual check valve. We used to estimate that it took 10mph to return the pistons. Until that , there will be some drag.
 
Just because the brake hoses are new doesn't mean they're good. If the internal lining of the brake hoses is swelling up it could be slowing the return of fluid to the master cylinder. It could also be an issue with the master cylinder, or the piston seals in the calipers themselves. With disk brake calipers the only thing that retracts the pads is the piston seals themselves, the rest is just knockback from the spinning rotor
 
Thanks everyone for your input, I greatly appreciate it!

I’m not crazy about it, and I’m not proud of it as it’s not stock, but I installed a return spring on the pedal and it seems to help the situation a good bit.

I noticed my brake lights were on after I shut down the car And got out. Thus, the pedal wasn’t returning fully for some reason and the Brake light switch wasn’t the problem.

I’ll have to dig in to the pedal and its bushing to check for wear/looseness.

It’s satisfactory for now but it ain’t perfect.
 
This is for sure not normal. You need to think "end to end." On the big pattern 73/4 brake conversions, there's a photo and paragraph in the manual of FITTING THE PADS to the caliper. This is rarely mentioned.

In no order
Sticking pads or other binding of caliper or pad fit, or even TAPERED DISK ROTORS

Hose problems
Problems in the dist block/ warning switch/ metering/ proportioning valve, AND I WOULD SPEND SOME TIME HERE

Master cylinder / boooster problems

Just about the first thing I'd do is get a helper and get "ready" so you can do this fast. Apply the brakes and confirm the problem, AKA attempt to rotate a jacked up tire. Quickly crack the line nut at the master and see if there is retained pressure. Move from there to the metering/ proportioning valve and crack both sides.

If you can, "end up" at the bleeder screw. If pressure shows up "at the wheel" but not on the master side of the proportioning valve, that may tell you something.
 
Chuck Henderson said:
Thanks everyone for your input, I greatly appreciate it!

I’m not crazy about it, and I’m not proud of it as it’s not stock, but I installed a return spring on the pedal and it seems to help the situation a good bit.

I noticed my brake lights were on after I shut down the car And got out. Thus, the pedal wasn’t returning fully for some reason and the Brake light switch wasn’t the problem.

I’ll have to dig in to the pedal and its bushing to check for wear/looseness.

It’s satisfactory for now but it ain’t perfect.
Click to expand...

It's not satisfactory. You've fixed the symptom, not the problem.
 
Correct; when we dry-out (tomorrow) I’ll have it in the driveway with the steering column bracket removed and the Wiring harness pulled out of the way so I can have a closer look at the brake pedal and switch.

Thanks again,
 
First, loosen the shouldered bolt between the M/C pushrod and the pedal, it sometimes binds and resists pedal return.
Don't forget to re-tighten, as nec. ! !
 
Do you have the adjustable push-rod for the master cylinder? Perhaps the locknut has come loose and the rod has slowly lengthened itself to the point where the brake light switch is applying the brakes even with your foot off the pedal?

That's just a thought. I had something similar at one point -- i noticed that the brake lights were staying on and it turned out my adjustable pushrod had the locknut come loose and the rod worked its way to being shorter. I put in some locktite when I adjusted it back to the right length, and then cranked the locknut too.
 
Last edited:
Chuck Henderson said:
also, do any A-Bodies have brake pedal return springs other than the spring(s) in the MC?
Click to expand...
Not that I'm aware of.

When you replaced the master cyl. Did you get a disk / drum or a drum / drum


If it's a drum / drum there is a residual pressure valve in the output port that you are using for the disk brakes.

That could cause the front disks to stay energized for some time.
 
Dana67Dart said:
Not that I'm aware of.

When you replaced the master cyl. Did you get a disk / drum or a drum / drum


If it's a drum / drum there is a residual pressure valve in the output port that you are using for the disk brakes.

That could cause the front disks to stay energized for some time.
Click to expand...
THIS
 
Dana67Dart said:
Not that I'm aware of.

When you replaced the master cyl. Did you get a disk / drum or a drum / drum


If it's a drum / drum there is a residual pressure valve in the output port that you are using for the disk brakes.

That could cause the front disks to stay energized for some time.
Click to expand...

Unless it's a really bad residual pressure valve, it should hold pressure for a lot longer than the 3-5 seconds that the OP describes...
 
It’s a front drum to disc conversion with a new MC, a stock push rod replacing an adjustable one and new hard and soft lines.

I thought that perhaps the original adjustable push rod was too long thereby keeping some residual pressure on the MC, but the stock replacement (one piece) push rod was actually a little longer. I’ve owned the car for two years and a previous owner had a bulky power drum unit installed. So, some parts (MC, push rod) weren’t stock.

The master cylinder is a front disc/rear drum unit with no residual pressure valve on the rear bowl (front brakes) but an RPV on the front bowl.

I replaced the brake light switch some time ago, and as you know it’s position is adjustable. I‘m going to look carefully at that next as it may be too far forward.

lastly, the manufacturer wasn’t too meticulous in the design and I had to grind down some metal off of the spindles as the calipers we’re hitting them.

lots of moving parts to deal with here and you’re all giving me a lot of good help. Thanks again
 
Don't know if it was mentioned, but one quick way to see if the master pistons are properly returning is tho have a helper apply the brakes, and observe the fluid in the master with the cover removed. Be careful of possible spurting fluid. Releasing the pedal should result in an almost instant visible return of fluid from the ports in the two sections
 
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