72 brake problems

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Phillip Tillman

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I have a problem with my brakes, I had to change my distribution block when I added rear disc brakes. When I went to bleed my brakes the brake peddle pumped one time and then became locked up and would not release. I went to 4 wheel disc brakes. I'm wondering if it may have to do with the distribution block be front disc to rear drums. There is no pressure or very little to the rear. All lines are hooked up to the same ports on the new distribution block, there is no fluid coming out of the rear brake bleeders.
 
So, what rear brakes are you running and what distribution block or combination valve do you have?

Which brakes are locked? Fronts? Rears? All 4?

If you have a distribution block, it doesn't reduce the pressure to the rears. If you have a combination valve like the disk/drums cars had, it does reduce the pressure to the rears. And disks take a higher pressure than drums typically do- but that doesn't explain the lock up. If the pressure was reduced too much to the rears you'd just have weak rear brakes.

Does this set of rear brakes have an integrated parking brake? Because a lot of the rear disks that have a parking brake function need to have the parking brake adjusted properly to work right.
 
I don't think so. I think it's just a manifold, with a pressure switch in it?
 
A proportioning valve has a feature that will shut flow of fluid off to the front or rear if a line breaks. Remove the switch from it and the plunger can be centered. There is a tool that will hold the piston centered while bleeding brakes. If your bleeding with a vac pump you can center it and bleed.
Go on youtube and search "proportioning valve reset".

Here's a decent video. Go to the 50 sec mark to see the shuttle valve.
 
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The new distribution blocks, my opinion, are junk, very poor machining. The 3 that I have had, all had issues. One of them did exactly what has happened to you. I was 30 miles from home and my rear brakes locked up, had to get trailered home. Distribution block was the culprit. Out of 3, I made one, but if it happens again, into the trash and I will go with a Wildwood or equivalent.
 
72bluNblu said:
So, what rear brakes are you running and what distribution block or combination valve do you have?

Which brakes are locked? Fronts? Rears? All 4?

If you have a distribution block, it doesn't reduce the pressure to the rears. If you have a combination valve like the disk/drums cars had, it does reduce the pressure to the rears. And disks take a higher pressure than drums typically do- but that doesn't explain the lock up. If the pressure was reduced too much to the rears you'd just have weak rear brakes.

Does this set of rear brakes have an integrated parking brake? Because a lot of the rear disks that have a parking brake function need to have the parking brake adjusted properly to work right.
Click to expand...
 
Ok, so the front brakes are locking, the rears are getting little/no pressure. This is after installing new rear brakes and either a new distribution block or combination valve.

A distribution block and a combination or proportioning valve are not the same thing. A distribution block is just a 4 way tee with a brake pressure switch. A combination or proportioning valve has a piston valve in it that lowers the pressure to the rear brakes and can close if there's a brake failure, like MoparMike1974 posted. Which one do you have? A distribution block is very unlikely to cause any issues unless it's plugged since it's just a tee. A proportioning valve can set up with the piston valve in the wrong spot and cause problems, which is why there's a bleeding procedure to adjust them.

Distribution block
Screen Shot 2022-09-16 at 9.52.24 AM.png


Proportioning/combination valve
Screen Shot 2022-09-16 at 9.52.44 AM.png


The inside of a combination valve (slightly different look, this is an XJ block but all the parts are the same)
XJ_Brake_Block_01_Notes.JPG.7bdef00277d10104cab73200ee47e139.jpg


I’m not familiar with a bkl250, gonna need more information than that. Manufacturer?
 
That bottom picture is the one that failed in my '69. The insides look a lot different, but probably do the same thing.
 
Also should note that you can pull that switch and it wont loose fluid, unless the seals are bad on the shuttle valve.
 
Brakes can hydraulically lock if the MC piston can't fully retract to expose the fill port. A common oops. If a booster, you must adjust the output tip length - youtubes, helps to have a measuring tool. If manual, people often adjust the pedal switch so the pedal bottoms out against the switch. No, the switch should only measure pedal motion and not affect it.
 
Did you pull the residual valve from the rear brake outlet in your master cylinder when you changed from drums to discs? It's located under the flare seat.
 
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