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Topic Review (Newest First)
11-12-2019 02:00 PM
PHWOARchild I read this early on and never got back here. On real old cars with drum brakes, sometimes the pads on the backing plates wear and the shoe gets caught and will not retract. The fix used to be get another backing plate or weld up the pad and grind it smooth. But what you did took care of the issue.
11-07-2019 12:42 PM
silver74vette You might try using body shims to move the M/C away from the firewall (effectively shortening the rod) to give you more play. What this sounds like to me is one of two things:
the rod is not fully releasing the piston keeping the equalizing port shut (this is normally seen on the back brakes)
The equalizing port is blocked.
10-11-2019 05:34 AM
rip-tide Pretty cool, the forum was all over the part comparability deal. Very glad to read you're officially rolling with a smile.

Thanks for letting us know about the results.

10-11-2019 04:31 AM
55 Tony Great job! Like said, good detective work, and thanks for letting us know the cure!
10-11-2019 04:15 AM
Ray Bell Yes, way too many people don't come back and let everyone know what happened.
10-11-2019 02:36 AM
brading Thanks for letting us all know how you found and overcame the problem. Good job you had a similar set up that you could check.
10-10-2019 11:48 PM
Ray Bell Who could ever guess that would happen?

Some great detective work there, congratulations. And commendation on your tenacity with it.
10-10-2019 08:15 PM
Finally discovered problem - Brake shoes

The problem was caused by the brake shoes not being made to the original factory specs. I ordered two sets from different suppliers and they were both made the same. I had to modify the shoes to make them work correctly.

I discovered the problem be examining the front brake action on another car with similar brakes while the pedal was depressed. The front shoe went forward and up but not as far as my car did, then returned easily to the home position. I took some digital calipers and started going over all my new brake parts and discovered the following two problems.

1. Near the top of the shoe on the back side was an indentation that was designed to rub on the backing plate. The old shoes were indented about 1/4 inch in a downward direction. The new shoes were only indented downward an eighth of an inch and when the shoe went forward and up it was catching on the edge of the backing plate. The only way I could remedy this is to spot weld a 16 gauge X 1/4 X 1 inch piece of metal to the indentation so it would ride on the backing plate correctly and not go over the edge. This worked great.

2. The second problem with the shoes was with the ear or the shoe that contacted the wheel cylinder was too short and allowed the brake shoe to go up too high when the brakes were applied. I spot welded the top of this ear to build this area up, filed it down smooth so that it was 1/8 inch taller than it was originally. This stopped the brake shoe from going too high and fixed that problem. After making these two modifications, I checked the brake action with the drums off and they worked correctly.

I put it all back together and drove the car 6 miles, then brought it back to the shop and re-inspected everything. The brakes worked correctly with no dragging. I drove the car for about a week stop and go in the neighborhood, re-inspected again and it all looks good and functions perfectly.

I want to thank everyone for their questions, comments, and advise. I followed everyone's advise and finally figured it out. This was the most difficult problem I have ever encountered.
09-25-2019 06:27 AM
Ray Bell Would it be possible to post a photo of the front brake assembly with the drum off?
09-24-2019 05:46 PM
Foperfoauto I wanted to clarify a couple things. Not always is the larger section of the Master Cylinder reservoir in the front. Yet, typically on the dual reservoir you will also notice the line that feeds the front calipers will be larger diameter. That is if you are trying to tell which feeds front and which feeds the rears.

As far as valves and functions, and I didn’t really explain this, the combination valve, often times just called the proportioning valve, actually isn’t.

The combination/proportioning valve in the front disk/rear drum system actually contains the metering valve, pressure differential switch, and the proportioning valve. The metering valve splits some pressure to the rears before the fronts start to grab. Otherwise it could cause the car to spin if the fronts lock before the rears. The idea is to have the brakes to apply smoothly and all wheels at the same time. The proportion valve helps to keep the rears from locking under hard breaking. The pressure differential switch alerts the driver to loss of balanced pressure in the system, due to a leak, or excessive wear.

Prior to current ABS this was the basic setup, and even though sometimes called the “proportioning” valve, the combination valve actually does more than what is commonly stated.
09-22-2019 03:54 PM
Foperfoauto The bottom of the shoes are not returning.

Why they are not, you need to check and figure this out. This is common in the Chicago area because of all the rust. At the shop (and this was eons ago) customers would set the emergency brake, and it would not release. The car would drive in reverse no problem, but not in forward. This is because when the shoes are applied they pinch. This is a pinching/wedging force, which is how drum brakes work.

AMC should be similar to the Ford system. The star wheel should be set up on the rear shoe. The adjuster only adjust when you apply the brakes going in reverse. If these are mounted backwards, which would be hard to do, the adjuster will rotate when you apply the brakes in forward motion. Not having the brakes in front of me…, I am not even sure if this would be possible. The shoes are applying and not returning to their relaxed state, this points to the edge of the shoe hanging up on something, or the return springs not installed correct.

Now if the MC is bad…, no pressure = no braking = none.

If you pump the brakes with the drum on, release the brakes, get out of the car and crack the bleeder and it just dribbles… This is exactly what it should do because the MC is in its stasis relaxed positioned. Now if you have someone in the car, pump the brakes and holding pressure, meaning applying foot pressure on the brake pedal and you crack the bleeder and it dribbles? You have plugged bleeder screw, no pressure coming from the MC, pinched line, or debris plugging something somewhere.

If the MC is building pressure when you press the pedal and there is nothing going to the WC (Wheel Cylinder) then there is a blockage between these two points. This is a very simple hydraulic system.

Now from what I just checked out, from 1971 AMC combined the proportioning and pressure valves into a single unit. Now I am not sure if that is what you have, because typically the “proportioning” valve is for cars with front disk brakes.

With disk, the front does the most of the braking. The rears anchor the car from spinning out because in physics, the rear is now going faster than the front. So the proportioning valve sends higher pressure to activate the calipers which are doing about 80% of the braking, the rears get enough line pressure to activate the shoes on the drums to slow the rear of the car down and keep it stable so you stop in a straight line. If you notice on a four drum system, both reservoirs of the master cylinder are the same shape and size (volume of fluid). On a Front Disk/Rear Drum system, the Front reservoir is almost twice the size of the rear.

Now I have had to do this in the past, and that is with the drum/disk system all setup installed like it should, pull the bleeder screw out and just hammer the brake pedal to the floor. If there is something in there debris, rust, piece of old piston cup, piece of broken spring, etc…, when you hammer the pedal down, you will blow it out or at least move it. Put a towel or something there so it catches the mess, and you will need to re-bleed that side. You can’t hurt anything, if its already broke, its already broke. Yet on a brake system you can't guess at this. You need to know what’s broken so it can be fixed, otherwise your looking for a crash…, or worse.

Your issue is mechanical. The shoes are not returning to a relaxed state. Its not E-Brake, because there is no E-Brake on the front. Its not line lock, because if it was when you crack the bleeder fluid would shoot out. Now if the bleeder is plugged, the fluid is still going to come out at the threads, because it’s under pressure. If it is not coming out then there is no pressure.

If you pump the brakes, get out of the car, foot off the brakes, crack the bleeder and it dribbles, that is what it should do because all the fluid forced into the wheel cylinder has already returned to the master cylinder.

Now on to the adjuster. It only adjusts when you apply the brakes going backwards. So back it off a couple turns, put the drum back on. Do the front shoes still lock up? Yes = shoes not returning.

Now drive straight backward about 10 MPH, apply the brakes. Do this about 6 times. The pedal should firm up and braking should be normal when driving in forward or reverse. Get on an area of empty pavement, gravel road, HS parking lot with no school in session, empty shopping center, whatever. Get going about 25 to 30 MPH and hammer the brake pedal down.

All the wheels should lock up. Get out of the car and look at the skid marks. They should all be similar. Which wheels locked up? Which ones didn’t? Are the fronts still not releasing?

Now the drums being out of round... If they are, under light application of the brakes you will get a pulsating pedal, but it won't cause the shoes to jam or not release.

The typical cause of out of round is hard braking when the drums are hot. Typically today, this is more on the rears. Someone drives home from work, store, what ever and the drums are hot. Hot Iron/steel is easy to bend. The person parks and jams on the E-Brake. So instead of applying just enough E-Brake for the incline the car is on, they jam it on full bore, and on a hot drum, they now just made a circle into and oval. They repeat this process every day,and so over time the drum is now a system of overlapping ovals. It is no longer a circle. However, it will not cause your shoes to lock up, it will cause a pulsating pedal, and poor shoe to drum contact, but won't cause the brakes to not release.
09-21-2019 07:28 PM
rip-tide It seems impossible both flex lines could have collapsed. Now acting like a check valve, and not letting the fluid to return.

I would also think the shoes could be locked up on jack stands. Unable to see how driving the thing changes the brake operation generally speaking. Unless using a higher pressure in the system causes trash of some sort to move and interrupt the fluid flow

I would try removing the brake line from a wheel cylinder next, one at a time

If the shoes return home the blockage, is between the MC and the wheel cylinder.

The front brake lines , they have a T block splitting the L&R wheels. Could a blockage rust, crud be fouling at that point

I believe you mentioned that new hardware, spring and such have been replaced?

This started after the new parts MC and so on were replaced ? If sure all those are in good shape, correct and installed correctly. The only thing that makes sense is some sort of fluid problem and it is not returning, back into the MC.

With the brakes locked, removing the brake line at the front of the system MC. Fluid did not push out as it should if the shoes are pulled home by the return springs. The fluid is being held up in the line, or T block.

One more I would find the T block remove the line that is supplying it, from the MC . Check and make sure it is clean first, all good move on to the other 2 supplying the wheel cylinders.

To clean, disconnect the wheel cylinders, blow air, from the MC end too the wheel cylinders. Compressor required, going to need some force, see what come out.

The wheel cylinders, you can push the pistons back without binding? Pretty crazy problem you have on your hands

Wish i was able to work on this as a first person.


09-21-2019 10:34 AM
55 Tony I think you got that backwards. Mechanical problem. No pressure when loosening the bleeder told the story.
09-21-2019 10:26 AM
Plumbing problem indicated with front brakes

OK, I followed rip tides advice and drove the car until the front brakes starting dragging. after I jacked the car up, I removed the brake line from the master cylinder that feeds the front brakes and the wheel remained frozen, so that indicates a plumbing problem according to your post.

I assume by plumbing problem you are talking about the brake line between the master cylinder and the two front brakes. I assume you are saying there is trash in the line somewhere.

What is the best procedure to clean the brake lines out? Thanks
09-20-2019 09:55 PM
rip-tide Have a look this is what goes on in the MC

Another good reference:

This one listen closely to what is said. ^ just because yours looks different means nothing the operation is the same.

I did not say crack the line I said remove the line.

I thought I read you had another MC? Did you bench bleed the MC?

Or install, fill and bleed the system? Bench bleeding is proper, best practice, however not necessary.

Think the MC is what needs to be addressed.

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