Dragging calipers on my '73 conversion

[Zymurgy]

Something just occurred to me and may be the cause of the problem. My dual brake booster is out of a '68 Chrysler, the master is for a '73. When I typed in a '68 master for disc brakes, I got a different master compared to what I have installed. What do you guys think, is this issue?

 

[mr. fix it]

I downloaded the manual, very nice to have the additional information. First the reservoirs were not over filled about 3/8" of air space, next I did the "Testing Master Cylinder" it seems to fail this test. No movement of fluid noticed definitely not a geyser of fluid squirting up into the reservoir. What exactly does this tell me? Bad master? View attachment 122672

There also seems to be very little pedal movement. My dual brake booster was sent to a highly recommend place to be rebuilt.

I then proceeded to take it out for a spin drove about 30 miles back roads, starting and stopping. Normal stopping, hard stopping and never got the the calipers to hang up. Personally the brakes felt perfect but there does seems to be very little pedal movement.

Additional trouble shooting out of '73 manual for reference.
View attachment 122673

I had a Duralast M/C that did the same thing
Hard pedal no fluid movement
The next one has been good

 

[Zymurgy]

I had a hell of a time getting the brakes bleed too. I ended up using my power bleeding vacuum tool. Should I get the '68 master instead this time, does it need to match the booster?

 

[70bigblockdodge]

Does the booster have a adjustable push rod? It may be too far out not letting the master release fully this could have cause the bleed problems.
This is a little adjuster on the master cylinder side of booster.

 

[Zymurgy]

Does the booster have a adjustable push rod? It may be too far out not letting the master release fully this could have cause the bleed problems.
This is a little adjuster on the master cylinder side of booster.

Thanks for the tip Dave. It looks to me from the picture that the push rod is threaded. It will be easy enough to check.

 

[cantflip]

Thanks for the tip Dave. It looks to me from the picture that the push rod is threaded. It will be easy enough to check.

View attachment 122689

That adjustment is what the test was for... doesn't sound like your compensating ports are open. That will cause your fluid to have no path back to the reservoir when un-applied and getting warm... recipe for a boil condition had you not felt the pull. The only thing I can think of off hand is the depth of the master cylinder piston may be different with the mix of booster/master or someone got happy playing with it... they usually are pretty close out of the box, but critical to check.

 

[Zymurgy]

Thanks again. So glad you pointed out that test. It seems to me like I should go ahead a order the master that is supposed to run with that booster.

I went ahead and ordered the correct master.

Thanks everyone for all the input. It all makes since now looking at the big picture. I feel confident this will solve my issue.

 

[Ross Wooldridge]

70bigblockdodge is right on the money.

Assuming for the moment that the master and booster are a balanced pair and all other areas of the system are in proper working order (compensating ports, non collapsed hoses, calipers all good etc etc), a properly adjusted booster pushrod should hold the master cylinder away from the booster about 1/16th of an inch when bolting the two together. At that point when you bolt it down, the pushrod applies the proper amount of "preload" to the front circuit. Any more and things start to hang up. Any less and there can be a delay in front wheel brake activation.

The pushrod is adjustible right at the very end of the pushrod... hold the long part of the pushrod from turning and use a small wrench to adjust the pushrod tip.

 

[Zymurgy]

Assuming for the moment that the master and booster are a balanced pair and all other areas of the system are in proper working order (compensating ports, non collapsed hoses, calipers all good etc etc), a properly adjusted booster pushrod should hold the master cylinder away from the booster about 1/16th of an inch when bolting the two together. At that point when you bolt it down, the pushrod applies the proper amount of "preload" to the front circuit. Any more and things start to hang up. Any less and there can be a delay in front wheel brake activation.

Nice information, I am pretty sure when I bolted up this master it moved more like a 1/4inch

 

[Ross Wooldridge]

Oy... that's definitely not good.

 

[thrashingcows]

I had the same exact issue with the brake heating up after I did the 73 disc conversion on my 68 T/C wagon. It turned out to be the pushrod adjustment, as mentioned. No problems after that.

Good luck!

 

[cantflip]

Thanks again. So glad you pointed out that test. It seems to me like I should go ahead a order the master that is supposed to run with that booster.

I went ahead and ordered the correct master.

Thanks everyone for all the input. It all makes since now looking at the big picture. I feel confident this will solve my issue.

Go ahead and try adjusting it (the push rod). This is all the same issue, and adjustment is usually easy. In many cases you can unbolt the master and pull it forward enough without disconnecting the lines or having to rebleed.

BTW, some shop manuals have a second test of loosen the master cylinder nuts 1/2" and retry the test, just to prove adjustment will allow the ports to be open.

 

[Zymurgy]

Thanks once again. After I slept on I thought the same thing. I will make a measurement to set it back to the factory setting if it doesn't work. I would love not to install a new master. I have felt I have done a lifetime of brake bleeding this spring.

 

[Zymurgy]

Thanks again everyone. I am not a mechanic as you can tell, but with the vast knowledge here I am learning. If I stand back I can understand the logic behind the trouble shooting. I learn something new it seems every day.

 

[traintech55]

Having done this to the same car, I remembered on thing. I originally made the mistake of trying to use the 72 - 73 Booster and master on my car, (yes found out the hard way about the shift linkage). When I did change to the 67 - 68 dual diaphragm booster and master I found out that while the piston in the newer master cylinder is flush with the back of the case, the older correct one is indented about a 1/4 inch, so again this is probably your problem. I would change the master cylinder to the correct one as I do not think you have enough adjustment on the booster shaft to give you the proper clearance. By the way I still have a new 72 - 73 booster and master cylinder for sale.

 

[Zymurgy]

Thanks for the heads up. I am hoping that they are about the same. They look similar in the picture.

This is the 68

The '73

 

[traintech55]

Thanks for the heads up. I am hoping that they are about the same. They look similar in the picture.

This is the 68
View attachment 122769

The '73
View attachment 122770

Yep, had the same problem when I did mine. The 73 would not fit on my 67 - 68 booster. That is why I got the master cylinder. Maybe you got (unlucky) getting yours to bolt on.

 

[Zymurgy]

Looks like I will be bench bleeding a new master. I got the '68 master today. The hole is nearly 1 inch deep. I am certain the booster can't be adjusted that much. Now I feel even more confident that this is the problem.

 

[mr. fix it]

Looks like I will be bench bleeding a new master. I got the '68 master today. The hole is nearly 1 inch deep. I am certain the booster can't be adjusted that much. Now I feel even more confident that this is the problem.

View attachment 123004

Glad you are confident that this is what the problem is.

 

[Zymurgy]

Finally got in my shop late this afternoon. Saw I could only move the booster push rod about a 1/4, so bench bleed my new '68 master.

The good, fastest I have ever bench bleed a master and didn't seem to introduce any air in the system in the master switch.

Took it for a test drive within about 10 miles both calipers started closing. 3 miles from home at 6 1/2 miles cool down intervals.

I never touched the push rod so I turned it all the way in. The brake pedal now feels right to me, before it would brake immediately, now there he typically movement before the brakes are engaged.

I ran out of time for a test drive but did the test with the master cover off with a piece of plexy glass. No helper, but when I looked at it there was fluid dripping from it so I believe it passed

I included 2 pictures one to see if you think he push rod is the right length after the rebuild. The second show when I first started getting resistance when I was bolting the master back on.