Disc brakes locked

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JGOODY

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I am completing a ground up resto of a 70 duster340. Front disc rear drum with manual master cylinder . I have bleed the master cylinder twice on the car with no air present. I have billed the entire system with noir present on the second bleed. I still have a soft brake pedal but more importantly both front discs are looked solid. Opening bleeder release wheel lock. Do I have a bad master cylinder or other. I am also suspicious of the secondary step down / delay valve that goes to the rear brakes. Could that cause the wheel lock also?

Thanks jeff
 
My hoses are brand new, but that is not that it can't be a possibility any other ideas
 
Yea, I have an idea.
The master cylinder actuator rod overadjusted?
It might be holding the seals on the piston beyond the fluid return ports not allowing the fluid to return and let the brakes let go.

Betcha 5 bucks. :)
 
Trailbeast thanks I will check it out, i don't recall an actuator rod adjustment though?
 
JGOODY said:
Trailbeast thanks I will check it out, i don't recall an actuator rod adjustment though?
Click to expand...

Some rods may be to long or to short in different cars. (even non adjustables)
If you want to verify, while the brakes are locked start loosening the master cyl bolts.
If the brakes let go, you know that's it.
 
A drum brake master cylinder will do this. Drums use a residual check valve and disc does not. I have seen this a few times.
 
chryslerfat said:
A drum brake master cylinder will do this. Drums use a residual check valve and disc does not. I have seen this a few times.
Click to expand...


Sorry, not correct. The residual valves don't hold anywhere near that much pressure, only a few pounds. In fact I just poked nails through both my valves to disable them, but prior to that I had run the car after the disc conversion for a couple of days with the valves in place

To troubleshoot this, you already know it's a brake line pressure problem, so just continue up the line. Get them locked up, then bleed at the distro/ prop block. This will now show if the hoses are what is blocking. Lock them up and bleed up at the master. If this releases them, most likely the pushrod/ linkage is not letting the pistons return.
 
Awesome suggestions , how can I verify if I have the correct master cylinder? What would be a good part number from say napa or advance.
 
I loosened the master cyclinder and also pulled back on brake pedal. Brakes still locked
 
I'm not convinced it matters if you get the pushrod issue fixed. I'm using the 4x drum cylinder on my 67 with 4x disc brakes.........73/4 Duster up front, and Linc Versailles out back. "Ruined" the residual valves, it stops great. I "will" have to watch fluid level, I'm thinking a whole 2--3 times a year, which all of us should be doing, anyhow.

Mine is the older style with the bolt that screws down the cover.
 
JGOODY said:
I loosened the master cyclinder and also pulled back on brake pedal. Brakes still locked
Click to expand...

Have you check your lines as Del suggested? Check the hard lines also. Have they been replaced? Do both front brakes lock up?
 
Thanks dart273 Iam using the same type master cyclinder with bolt in center. Iam going to try bleeding at the prop valve as you suggested. I am Thinking I Have the wrong master cyclinder
 
All of my lines , hoses ,master cycl, pro portioning valve , calipers everything is new
 
pump up your pedal and get the front brakes to lock. then crack open the lines at the master. If you have pressure there. It's the master.

Or a pedal/pushrod adjust.
 
I' m not sure what you mean by pressure at the master? I am convinced if I crack the lines at the master it will free brakes up . I disassembled a caliper last nite the piston was extremely tight in the bore if I put air pressure thru the bleeder it pops right out but putting it back in the bore it is super tight?
 
Again..............you said releasing pressure at the caliper releases the brake. This indicates a pressure problem. If you bleed it at the caliper, it could be the hoses. If you crack the nut at the line at the master cylinder, and this ALSO releases the brake lock, then it cannot be the hoses---it must be pressure retained in the master.

Again............if the piston in the master piston is not returning, either due to wrong internal parts as in rebuild/ rebuilt, or wrong combo of master and pushrod / linkage, then that will hold pressure and lock them up.

I did this years ago on a 57 chev. Got the master together wrong, the master could not fully return the piston. Drove the car for several blocks, and in about 4 blocks, corners, stopping, etc, the brakes were dragging BAD

Down in your master there should be what is called a "compensating port." The piston MUST return fully to release this pressure.

Also pull up on your pedal and "feel" the linkage. You should have "some play" with the pedal all the way up
 
You asked about verifying you have the correct master cylinder and NAPA or Autozone.part numbers. I went through getting a new master cylinder a month agao and decided on Rockauto as the Raybestos matched EOM very close…and the brake line connections were the same size as what I had…just wanted to bolt it on.

IF you decide on a new master cylinder, you can measure what size nuts/threads you have at your master cylinder and see if an auto store near you can match it up to a cylinder they sell for disc up front and drums in the rear…the folks at the stores near me couldn't guarantee what they had for sale would match up. I decided on Raybestos #36426 at Rockauto for $66--correct for a 71 340 Duster with Disc up front and drums in the rear. I looked for a 70 340 Duster with disc up front and rear drum at Rockauto and there are Cardone for $40ish. You can look at the pictures and connection sizes and hopefully pick one that won't require you to redo your brake line to match….although if your lines were previously changed you probably want to get the right master cylinder and match the lines to it. I'm certainly no brake expert…you might be fine with a drum brake cylinder, but I went with EOM repo correct to my configuration. Good luck!
 
What kind of prop valve are you using...an aftermarket adjustable one or a stock style repro? There have been a lot of issues with the stock style repro prop valves. They were machined wrong and lock up the brakes. I don't know if this was ever fixed or if there is still a problem with them.

Push down on the pin that sticks out the top of the valve and see if that relieves the pressure.(if you are using a stock style valve)
 
One quick question. Are you still using a drum brake proportioning valve?

You Have to swap out the valve when you change from drum to disk brakes!!!!

The drum brake valve holds pressure on the front brakes so that they start grabbing before the rear brakes. Other wise you end up with a ride that spins you out every time you hammer the brake pedal hard.

I had the exact same issue the O.P. is describing when I did my first drum to disk swap like thirty years ago.
 
Mopar Marv said:
One quick question. Are you still using a drum brake proportioning valve?

You Have to swap out the valve when you change from drum to disk brakes!!!!

The drum brake valve holds pressure on the front brakes so that they start grabbing before the rear brakes. Other wise you end up with a ride that spins you out every time you hammer the brake pedal hard.

I had the exact same issue the O.P. is describing when I did my first drum to disk swap like thirty years ago.
Click to expand...

Geezuz. NOT TRUE. The drum brake "device" is NOT a prop valve. It is nothing more than a sliding piston which activates the warning light switch in case of pressure difference between front / rear, indicating a leak in one end.

The drum brake device would have nothing to do with the front brakes locked.

THIS is what drum brake cars have......a hydraulic piston operated switch

71-drum.jpg
 
67Dart273 said:
Sorry, not correct. The residual valves don't hold anywhere near that much pressure, only a few pounds. In fact I just poked nails through both my valves to disable them, but prior to that I had run the car after the disc conversion for a couple of days with the valves in place

To troubleshoot this, you already know it's a brake line pressure problem, so just continue up the line. Get them locked up, then bleed at the distro/ prop block. This will now show if the hoses are what is blocking. Lock them up and bleed up at the master. If this releases them, most likely the pushrod/ linkage is not letting the pistons return.
Click to expand...

Sorry 30 years as a certified mechanic and working in shops my life I have seen the valves cause this. They can hold enough to stop a rotor from turning by hand but you could put it on the ground and drive it until it ignites the pads from dragging.
 
67Dart273 said:
To troubleshoot this, you already know it's a brake line pressure problem, so just continue up the line. Get them locked up, then bleed at the distro/ prop block. This will now show if the hoses are what is blocking. Lock them up and bleed up at the master. If this releases them, most likely the pushrod/ linkage is not letting the pistons return.
Click to expand...

Crack the lines at each connection upstream till the brakes release, that will tell you where the problem is otherwise you are just guessing.
 
Everyone thank you !! I will keep you posted on what I find out tomorrow
Jeff
 
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