Front disc brakes locking up, 72 Duster

[oldirish]

Looking for ideas.

The front brakes get very tight while driving. Off the ground you can't turn the wheels by hand.

Once the car cools off, 20-30 min, the wheels turn.

With the brakes locked If you break the bleeder or hose connection the brakes release.

Thanks for any help.

 

[mopar head]

If your hoses are old, they can collapse and cause that.

 

[72bluNblu]

Which disk brakes? Original KH disks with 4 piston fixed calipers? 73+ disk conversion with single piston sliding calipers? Some aftermarket conversion?

Original combination valve? Distribution block from a drum conversion?

Rear brakes working at all? Drums still?

If your hoses are old, they can collapse and cause that.

They sure can. Doing it equally on both sides is a bit of a trick though (but not impossible). Still if it’s both fronts equally I’d be looking at the system before it splits to start with.

 

[oldirish]

If your hoses are old, they can collapse and cause that.

That was the first thing I checked, cracking the connection at the top of the hose releases the brakes so they don't appear to be the source.

 

[moparker]

Are the rubber lines old? They can look fine but, internally be shot.

 

[oldirish]

Which disk brakes? Original KH disks with 4 piston fixed calipers? 73+ disk conversion with single piston sliding calipers? Some aftermarket conversion?

Original combination valve? Distribution block from a drum conversion?

Rear brakes working at all? Drums still?



They sure can. Doing it equally on both sides is a bit of a trick though (but not impossible). Still if it’s both fronts equally I’d be looking at the system before it splits to start with.

Single piston Original Mopar parts probably from a 73? Not sure about the distribution block.

Having both sides lock up and feeling the same is what puzzles me.

Also freeing up after it cools down.

Could header heat cause this?

 

[oldirish]

Are the rubber lines old? They can look fine but, internally be shot.

They don't look old they're in good shape. I pulled one off to replace it but I could blow through it just like a new line.

 

[Professor Fate]

Manual disc or power assisted?
If manual, you could have the MC pushrod adjusted a bit too long.

 

[oldirish]

Power brakes, new booster.

 

[72bluNblu]

Single piston Original Mopar parts probably from a 73? Not sure about the distribution block.

Having both sides lock up and feeling the same is what puzzles me.

Also freeing up after it cools down.

Could header heat cause this?

It’s not header heat.

The hoses can be brand new and separate internally, but that’s usually a “one side” problem.

Power brakes, new booster.

If this started after the new booster install, the adjustable push rod in the booster is the wrong length.

 

[oldirish]

It’s not header heat.

The hoses can be brand new and separate internally, but that’s usually a “one side” problem.



If this started after the new booster install, the adjustable push rod in the booster is the wrong length.

Sounds like a good place to try next.

 

[oldirish]

The brake pedal isn't tight, there some loose travel at the top.

 

[oldirish]

Image of proportioning valve

 

[72bluNblu]

Sounds like a good place to try next.

View attachment 1716348292

Oohhh. So that’s a brand new booster and GM style master cylinder. So some online kit deal then?

Is this a brand new install? As in, did this car have working disk brakes before and you just added the booster and master cylinder? Or did this car not have working disk brakes before?

The brake pedal isn't tight, there some loose travel at the top.

That’s a problem.

Image of proportioning valve

View attachment 1716348298

Looks like a 73+ style factory combination valve. With some definitely not factory brake lines.

 

[67Dart273]

CHECK THAT THE MASTER IS FULLY RETURNING. If the pushrod is too long or the master is somehow otherwise being pushed towards the front some, the return ports in the floor of the reservoirs will NOT return the fluid, and the system will hold pressure.

YOU ALREADY KNOW it's a pressure problem as you said you cracked the tubes and the brakes released. There is only so much in there that can do so. The master not returning as I mentioned, some problem with the prop valve (new does not mean "good") or bad hoses as mentioned.

 

[Professor Fate]

Sounds like a good place to try next.

View attachment 1716348292

Single piston Original Mopar parts probably from a 73?

Maybe your spindles/calipers are, but the stuff you show topside definitely aren't Mopar parts.
They look Chevy to me, probably came as part of a conversion kit, like this:
62-74 Mopar Chrysler Plymouth A body power front disc brake conversion 5 on 4.5 | eBay

It comes with a variety of parts from various sources, and if you notice, it comes with a few different sized linkage arms in an attempt to make it fit as many different vehicles as possible.
Your problem could be as simple as having the incorrect linkage arm installed, or as involved as having a bunch of mismatched parts.
Those kits have been made to work, but they have a rather extensive history of issues. Probably every third post about braking problems involves these kits.
First course of action would be to make sure it's installed and plumbed correctly- it appears you have a Mopar type combination valve, but you need to make sure it's hooked up correctly, and the same goes for the master.
By the way, these kits come with a separate proportioning valve mounted directly underneath the Chevy master, and if that was retained and also plumbed into the Mopar combination valve on your inner fender, could be causing some of your problem...

 

[Dana67Dart]

My money is on the pushrod in the booster is not adjusted correctly, too long

 

[MV8]

Looking for ideas.

The front brakes get very tight while driving. Off the ground you can't turn the wheels by hand.

Once the car cools off, 20-30 min, the wheels turn.

With the brakes locked If you break the bleeder or hose connection the brakes release.

Thanks for any help.

REPLACE the phenolic caliper pistons with steel or a hybrid phenolic with metal. Phenolics are ok when used regularly but they absorb moisture that increases the diameter and expands when the pistons get hot. The moisture cannot easily escape quick enough to prevent piston expansion. This has been a problem for all the domestic mfgs, especially RVs that sit for long periods.

 

[Professor Fate]

REPLACE the phenolic caliper pistons with steel or a hybrid phenolic with metal. Phenolics are ok when used regularly but they absorb moisture that increases the diameter and expands when the pistons get hot. The moisture cannot easily escape quick enough to prevent piston expansion. This has been a problem for all the domestic mfgs, especially RVs that sit for long periods.

Good point; but if his pistons were seizing, cracking the bleeder or hose open wouldn't release them like the OP says they do- they'd still be stuck until they cool down.

 

[Murray]

Wrong M/C having a residual check valve in the front line.

 

[Professor Fate]

Wrong M/C having a residual check valve in the front line.

Good thought- It does appear to be a drum brake master, although I'm not as familiar with GM parts. If the residual valves remove as easily from the GM masters as they do from Mopars, that could be it. Or the front brakes are plumbed to the wrong MC reservoir.

 

[crackedback]

Slowly loosen the master fro the booster, see if the brakes release. Will help identify if the push rod may be too long.

 

[Daves69]

Slowly loosen the master fro the booster, see if the brakes release......

This^^
You might also consider some temporary shims/spacers prior to that.