Front brakes won't release

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moparfanatic56

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This a spinoff on my post about the brake pushrod length. I purchased a front disc brake conversion kit from toms classics which according to them it is from a 76 to something dodge aspen for my 72 dart. I have all new lines from inline tube, been thru 3 master cyclinders and 2 distribution blocks. The front calipers (both sides) will not release or release very very slowly, I have cracked the bleeder loose and the rotor will turn with some resistance. I also cracked the main line on the master cyclinder with the same results and I also checked the brake pedal and it is not being stopped by the brake light switch plus I can pull it up approximately 3/4 to 1 in so I am guessing the push rods is the correct length. So what am I missing are both of the new rubber lines possibly collapsing or something else. I am completely stumped. Thanks
 
Do you know if the master cylinders you’ve purchased are specific to front discs? Drum brake type will have a residual pressure valve designed to maintain pressure…I’m thinking this is your issue.

Sometimes an easy way to tell is that the disc master cylinder will often times also have a larger reservoir for the front/disc brakes since they compensate for pad wear over time vs a shoe setup that has to be adjusted manually to compensate for the lining wear.

Here’s a quick side by side for comparison:
1753738855326.png
 
One other thing i forgot to mention is I am on my 2nd pair of calipers also. I pulled up the dr diff conversion kit and it is almost identical to the kit I have now, the slider type single piston.
 
One thing I saw on a car with this issue. He installed stainless lines and had a hard time sealing them. Over tightening squashes the brass nipple. the flare just squeezed it together and almost closed the opening.
 
Oldmanmopar said:
One thing I saw on a car with this issue. He installed stainless lines and had a hard time sealing them. Over tightening squashes the brass nipple. the flare just squeezed it together and almost closed the opening.
Click to expand...
Ì am assuming you mean the nipples on the Mc. If this is the case wouldn't that cause a reduced flow when bleeding the brakes or not. I have a good steady stream when bleeding them.
 
To be clear, disc brakes don't have any form of "return" spring, - the pads merely relax without pressure, the rubber seal is supposed to withdraw the piston a coupla thousandths, the disc may still slightly drag, often relying on any disc "warp" to knock the pistons back to actually get totally free/no drag disc movement.
Not sure that helps,
Good luck.
 
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You should have put this in your other thread. The brake pedal does not always tell you. Loosen and or REMOVE the master from it's mount to make sure the piston is fully returning

Did cracking the line fittings IMPROVE the release?

I know you claim this is a correct cylinder but in this day and age I would suspect ANYTHING. Make CERTAIN whichever end of the master the fronts are connected to do NOT have residual valves. You should be able to tell by gingerly probing with a small nail / brad. I would clip the point and then grind / sand the nail to a flat/ rounded end.

If it's a restriction in the tube system the ultimate test would be to have someone pressure them up, then release a bleeder at one caliper

Other than that are these rebuilt or new? If rebuilt, "suspect."

ALSO I am not familiar with the pads on these later brakes. Are they similar to the 73 brakes? If so, READ THE factory manual. Some pads need to be "seated" with a mallet. They can bind and maybe cause the piston to bind.
 
That was one of the other tests that I tried was loosening the master cyclinder and there was not any pressure release on the calipers.
When I pulled the dr diff kits it did show having to bend the tab for the outward pads some, but I have not tried that yet.
I do not know if they are new or rebuild, but i am going to assume they are rebuilt as most parts you get anymore are plus with the calipers I had to turn in the old ones to get my core charge refunded.
There was relief on both the bleed valve and the main line at the Mc at least to the point that I could turn the rotor with a little effort.
 

Then if there is hydraulic relief, that for the moment eliminates caliper/ pad binding. I would screw with pressure relief and try to determine if there is a tube/ hose or distro/ prop valve problem, and work more with the master.

You may have been misled about the residual valves, but I don't think they would cause such lockup
 
If s/s lines, post #4. These cars used ' Bundy' tube for brake, fuel lines, etc. It is a special soft, compliant steel, that will flex [ seal ] easily with out excessive. The matching components [ M/c etc ] are designed with components [ flare seats etc ] to work with this. Then some idiot, with no material or engineering experience, sells super hard s/s line that deforms other parts....
 
Like everybody else is saying, my guess is that the caliper pistons are not able to force the fluid back into the reservoirs.
My first go-to would have been pushrod length, and
Second would be plugged compensating ports, and
Third would have been the square-section caliper seals, in corroded grooves.
But since this is an ongoing problem, My question is in regards to plumbing;
In the factory system, the front brakes are plumbed to the REARMOST chamber, via the "top" of the Combination valve, which is the opposite end of the Proportioning part, and thru an OPEN port without a residual valve.
Jus saying.
 
Here are some pictures don't know if they will help. I have checked diagrams on the plumbing for the distribution block and everything looks correct. I just loosened up the Mc and slid it out a little ways but there was no change or relief on the calipers. I guess what is puzzling me is after a minute or two the caliper will release enough to turn the rotor.. I also loosened the bleed valve and it did relieve pressure and then I broke the hard line and rubber hose connection and it also released the caliper somewhat.

20250729_105931.jpg


20250729_105856.jpg


20250729_105812.jpg
 
No there is a slight jog in the tube and apparently it's a shadow making it look like it's bent. Good eye though always nice to have more eyes looking especially for an old blind guy like me lol
 
Take the lines off the distribution block . Look in and see if the nipple for the flare is squeezed together from over tightening the lines. Also check the hoses where the lines attach. The new after market parts are not made from the same quality metal they use to be.
 
Okay,,,you know it’s not the calipers .
It can only be the rubber lines,,,loosen and see if they release .
If so,,,,then the distribution block,,,loosen lines there,,,first out port,,,then the in port .
After that ,,,MC lines .
There is not much left really .

Tommy
 
Loosen the tube for the front right at the master. If they release, it's not hoses, or tubing, or prop valve/ distro/ warning switch. it is the MASTER. And as I suggested, CHECK whether that master has a residual valve, and if not, it has GOT to be something right there. Hell it might even be a defective master, I don't know what--Junk in there/ sticking piston? Mismatched parts? No idea.
 
67Dart273 said:
Loosen the tube for the front right at the master. If they release, it's not hoses, or tubing, or prop valve/ distro/ warning switch. it is the MASTER. And as I suggested, CHECK whether that master has a residual valve, and if not, it has GOT to be something right there. Hell it might even be a defective master, I don't know what--Junk in there/ sticking piston? Mismatched parts? No idea.
Click to expand...
OK, I did a little research on this and there is supposed to a residual valve for the rear drum but not one for the front disc if the Mc is above the calipers and a residual valve for both if the mc is below the calipers. I also learned that if there is a residual valve the fluid will just trickle out of both ports on Mc with the lid off and if no valve it will flow out steady. I tried this on one of the Mc I took off and the fluid came out of both ports at practically the same rate. I guess this means I have the residual valve and need to find one without it.
 
Here a pic of where the residual valve is located in the m/c.
Use a self tapping or similar screw to pull the brass seat outta the hole, remove rubber, push brass seat without rubber and spring back in.
Tightening the tube fittings till tight.
Or shove a wire or similar into hole, piercing the rubber valve. An

Good luck

IMG_20240418_094319.jpg


IMG_20240418_094231.jpg
 
And I already suggested, you can carefully probe the master fittings and figure out if there is a residual valve in there
 
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I took a small drill bit and used the shank end to probe and there was no resistance or anything that felt like I was hitting a rubber seal. I took one of the other Mc and pulled the brass nipple out and there was not a residual valve there. I also checked the port for the rear brakes and no residual valve in there either that I could feel. I am now leaning towards either the nipples in the distribution block being squeezed almost shut or the rubber lines collapsing
Click to expand...
 
When u loosen off the bleeder screw or the brake hose and even though the caliper relaxes doesn’t mean the hose is any good. Lots of times they have good flow while bleeding but the fluid can’t return cause the hose is collapsed inside. Kim
 
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