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Next problem, Grabbing rear brakes

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Category: The Garage
Forum Name: Suspension, Steering, Brakes & Wheels
Forum Description: What makes it stop, turn, and smooths the ride
Printed Date: Dec/06/2022 at 12:34am
Software Version: Web Wiz Forums 12.03 -

Topic: Next problem, Grabbing rear brakes
Posted By: Verno
Subject: Next problem, Grabbing rear brakes
Date Posted: Aug/23/2022 at 9:46am
1963 Ambassador 990 wagon, power drum brakes.

Think I may have resolved the engine stalling issue with a carb rebuild, and also discovered a leaking fuel pump which has now been replaced.

Next problem to surface is that when i backed out of the garage to test out the carb and fuel pump fixes, the rear brakes grab instantly at the slightest pedal pressure. Pulling back in garage was same, instant locking.   
Brakes seemed to work fine prior.

Have all drums off and no fluids on shoes.  TSM mentions potential control valve. 

Any tips for further diagnosis before digging any further?

Thanks in advance


Posted By: Skwishy
Date Posted: Aug/23/2022 at 11:08am
the rear adjuster might be out too much. taking the covers off the brakes and bring the drum adjuster screw back in

'68 AMC Ambassador SST 390, '68 AMC Ambassador 290, '81 C-10 shortbed, '79 C-30, looking for ramblers & rouges with a manual trans.

Posted By: FleetMaster
Date Posted: Aug/23/2022 at 11:22am
Crack a bleeder open one one side and see if still pressure and shoes retract slightly.  Your description suggests both sides grabbed so unless both adjusters set same, dead on same.  one side should grab first.  My thought is the hose is internally damaged and although master cylinder is released, hose prevents backflow of fluid and release of braking pressure.  All but the pressure setting of the residual pressure check valve at the master cylinder should be present, 8-10 psi IIRC

"If the women don't find you handsome, they can at least find you handy" Red Green

Posted By: Heavy 488
Date Posted: Aug/23/2022 at 11:22am
How long was it sitting since the brakes were replaced?
What's the humidity been like?

Posted By: Verno
Date Posted: Aug/23/2022 at 3:11pm
A little more detail: 

Drove the car around the neighborhood a couple weeks ago and brakes worked OK. Car has been stored inside since we got it a few weeks ago.  

There’s no real brake drag unless there’s pressure on the pedal. In neutral on jack stands can turn the wheels pretty easily. Just what seems to me to be normal drag from the diff. 

I’ll try cracking rear bleeders and see if that may release any residual pressure. 

Whoever put the last brakes on was a hack. Front had no auto adjusters and the spring was just put across the bottom of the shoes.  Also found that they put both trailing shoes on the right side and both leading shoes on the other. 

Today see that rear has missing auto adjust mechanism also. 
Going to go ahead and replace shoes and install auto adjusters. 

Posted By: 79 SPirit
Date Posted: Aug/23/2022 at 3:39pm
A bad booster will do that. How old is it?

Posted By: Verno
Date Posted: Aug/23/2022 at 3:49pm
No ideas on booster age. 
What is the failure mode for a booster? 
Diaphragm leak, sticking valve, ???

Would the front/rear separation valve in the master
cylinder cause the problem?

Posted By: Verno
Date Posted: Sep/07/2022 at 7:08am
The brake booster saga continues. 
After pulling the booster for rebuilding, thought I would check the clearance from booster shaft and mc.  
Assuming it should have been .020-.060.  Turns out it’s ~.300. 
Would explain why the bakes went from a little touchy to instant grabbing last time I backed out of garage.  

Posted By: ramblinrev
Date Posted: Sep/07/2022 at 8:35am
You said the brakes seemed to be installed poorly.
Double check that the primary shoe (the one with the shorter length of lining) is facing the front of the car, and that the secondary shoe (with the longer length of lining) is facing the rear. If those are not correct, it can lead to brakes grabbing too!

74 Hornet Hatchback X twins (since 1977)
62 American Convertible (still worth the $50 I spent in 1973!) AMCRC #513, AMO #384
70 AMX 360 4-speed (since 1981)

Posted By: Verno
Date Posted: Sep/22/2022 at 9:45am
Sent my brake booster off for rebuild and it was received at power brake exchange on Sep 10th and got call yesterday that it was done and being shipped back. 
Pretty happy with the turnaround as I was anticipating 3 to 4 weeks. 

Posted By: Trader
Date Posted: Sep/23/2022 at 3:19pm
Another thing to check is that the emergency/parking brake cables are loose, lubricated and retract fully.
On many automatic cars people rarely use them and once they do they just seize up.

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