brakes locking up. What did I do wrong?

[daves68]

Stats: 68 Charger with factory 10 in manual drum brakes. Switched to 11 in all around and added a booster. Everything esle stayed the same Still using manual master cyl.   Now there is marginal braking and a hard pedal until about 1/3 of pedal travel. Then all 4 wheels uncontrollably lock up. Can't drive like this! . Would like to put on a front disc system but can't seem to find one that will fit  my 14" wheels. All constructive critiques cheerfully acceptes.

[b5blue]

Dave, refer to Mopar Action Disco Tech article online, more there then I can tell you for your 14" wheels. Your current problem can be air in lines, bad booster push rod adjustment between booster and master, wrong or bad booster or linkage and possibly the shoes are not adjusted out till just barely touching the drums for new seating/break in. I just changed to manual brakes when my booster failed and like it better. Did you use factory or aftermarket linkage for the swap? The feel is completely different between the PB and manual systems. 

[elacruze]

I'm no specification expert, but the manual master cylinder is likely much smaller than the power MC, giving a lot more leverage. Combined with the Vacuum booster it's possible you're just overpowered. That explains the lockup, but not the late action of the booster.

Using my imagination (never assume that corresponds to reality) I can see;

New shoes/drums, they're weak because they're not broken in yet.
Booster/MC mismatch, could have something to do with booster timing.
Booster/pedal rod mismatch, could have something to do with timing.
Brake shoes installed backwards, causing sudden lockup; I'd verify this no matter what else you do. Long shoes in the rear, short shoes in front.

You have a lot going on there, which makes Telephone Diagnosis particularly difficult.

[daves68]

I checked the orientation of the shoes and thats OK. The under dash linkage was donated by a 75 Cordoba. Will try bleeding again.  Thing is that it worked fine in the manual mode. It was only after addition of the booster that the excitement started. Still would like to find a disc swap for my 14" Spent too much on wheels/tires to dump them. I had a 74 Charger at one time and that had discs on 14 wheels. I have heard of guys doing this swap, may have to hit the yards for parts.

[elacruze]

Small discs should be very easy to find, from Darts or such.
www.junkyarddog.com

[John_Kunkel]

What is the history of the booster? New? Used?

Touchy power brakes can be caused by a misadjusted pushrod or a faulty reaction valve in the booster.

[daves68]

The booster is a reman from Rockauto.com

[alcusswhen]

I checked the orientation of the shoes and thats OK. The under dash linkage was donated by a 75 Cordoba. Will try bleeding again.  Thing is that it worked fine in the manual mode. It was only after addition of the booster that the excitement started. Still would like to find a disc swap for my 14" Spent too much on wheels/tires to dump them. I had a 74 Charger at one time and that had discs on 14 wheels. I have heard of guys doing this swap, may have to hit the yards for parts.

The booster you used is no good. I just had the same problem changing over my 73 Charger. It near drove me nuts.
My swap stuff came from a 79 Cordoba. I bought a booster from Summit for my 73 still used the Cordoba linkage but it solved the problem.

[HOTROD]

I checked the orientation of the shoes and thats OK. The under dash linkage was donated by a 75 Cordoba. Will try bleeding again.  Thing is that it worked fine in the manual mode. It was only after addition of the booster that the excitement started. Still would like to find a disc swap for my 14" Spent too much on wheels/tires to dump them. I had a 74 Charger at one time and that had discs on 14 wheels. I have heard of guys doing this swap, may have to hit the yards for parts.

The booster you used is no good. I just had the same problem changing over my 73 Charger. It near drove me nuts.
My swap stuff came from a 79 Cordoba. I bought a booster from Summit for my 73 still used the Cordoba linkage but it solved the problem.