Rear wheels spin with brakes pressed

91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
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North East
Gents, here’s another weird one: maybe someone can provide some insight? The Car is a 1991 5.0 LX with factory brakes. I changed the rear drum brakes yesterday. Springs, shoes and drums. Everything works fine. However when the rear is up on stands/Jack and in drive with the rear wheels spinning, when the brake is applied the wheels stop immediately but then begin to spin at a medium rate even while the pedal continues to be pressed. The emergency brake works 100%. Stops and doesn’t start to spin again. There are no brake cylinder or master cylinder leaks. The brake adjustment seems to be fine because I made it tight enough to make it hard to pull the drum off and hard to rotate the tire by hand so I don’t think it’s an adjustment issue. Perhaps this is normal? Curious if anyone has any insight as to why the rear brakes would stop the rear tires 100% for a few seconds but then they start to spin again even with the pedal still pressed while the rear is elevated? PS: I’m not sure if it always did this or if this is the result of my brake job yesterday. Maybe someone can try this and see if yours starts to spin again after stopping when the pedal is still pressed?
 
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Kid wita 5oh

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IF the adjustment is indeed correct, it sounds like you're bleeding off pressure somewhere....recheck for leaks(peel back the wheel cylinder boots a little if there's no external signs)
How is the pedal feel? Hard, soft, normal? Does it start to sink if you keep your foot on it?
MC could be bleeding by internally or out the back into the booster....
 

90sickfox

Wasn't a pretty sight...and I've got big hands
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The rear wheels should not spin with the pedal pressed. Sounds like you might have locked up wheel cylinders or a restriction in the line to the rear. They would give a firm pedal. A soft pedal with this symptom would be air or bad master cylinder.
 

91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
53
North East
Thanks for the info. I will check the line underneath for restrictions first. It sounds like I need either new brake cylinders or a master cylinder. These Parts seem to be inexpensive enough so instead of spending more time on this I may as well just change those parts and assume that will be the fix. Also nice to know after 30 years that the cylinders are new.
the pedal doesn’t get mushy when it’s down. It feels normal and maintains the steady feel.
before swapping the parts I will check the line for restrictions. At least I know this isn’t normal now. I didn’t think so because it defies common sense but on the other hand, ya never know.
thanks guys
 

General karthief

wonder how much it would cost to ship you a pair
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I will add:
pump the brake peddle a couple times then put it in gear, holding pressure will the wheels start turning again? Can you feel the peddle move?
 

7991LXnSHO

wanna catch the space herp
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Checking the lines and hoses for restrictions is a good safety idea, but does not make diagnostic sense for this odd issue.
I am assuming the OP is not power braking on jacks. :)
The pressure is initially there to stop the wheels. As long as the pedal is pressed, the line pressure should not be reduced. The fluid has to be leaking somewhere. If the level in the reservoir is not going down, and there are no places at the wheel cylinders, (even still in the rubber caps), or lines where the fluid is outside of the system, and it is not filling the booster, it must be leaking internally. That often is in the master cylinder. The system has a dual fluid circuit to help keep half the brakes when the other half fails. In this case, I believe one half of the master cylinder is leaking internally.
If you do not know how old the wheel cylinders and hoses are, replacing them before bleeding the whole system is also a good idea.
 
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7991LXnSHO

wanna catch the space herp
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I will add:
pump the brake peddle a couple times then put it in gear, holding pressure will the wheels start turning again? Can you feel the peddle move?
Please do this, and I hope your response to the General’s test is conclusive.
There might be problem in the proportioning valve, but that leak will usually show up externally.
 

Mustang5L5

Put lubricant all over the balls
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Definitely bleeding pressure off somewhere. You actually don't know if the front brakes are affected because they aren't being driven like the rear wheels are.
 

7991LXnSHO

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Definitely bleeding pressure off somewhere. You actually don't know if the front brakes are affected because they aren't being driven like the rear wheels are.
Good point! Unless the front was also jacked up and torque was applied to check, it could be both ends.
 

91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
53
North East
oh man, I hope this isn’t on both ends but your right, it could be! Dangerous situation and the scary part is that I would have never known if i just dropped the car and went for a test drive and figured everything was fine until at some point the brakes gave out. No bueno.
Well I just got home from a long day and it’s a bit late to start playing now so tomorrow i will check a few things out that you guys mentioned especially the generals test. I appreciate everyone’s input on this. Looks like a Master Cylinder at minimal is in my near future.
 

7991LXnSHO

wanna catch the space herp
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Curiosity - Why did you change the rear brakes? Did you notice only the fronts were working, was it noise, or was it good maintenance?
 

91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
53
North East
Curiosity - Why did you change the rear brakes? Did you notice only the fronts were working, was it noise, or was it good maintenance?
Maintaince only.
I bought the car in Dec 2020 so without knowing the last time someone tuned it up, changed the fluids, changed the brakes, etc, I am slowely banging these items out so that I know exactly what’s new and when it was replaced and how many miles on the car at the time of replacement, etc.
Although there didn’t seem to be anything wrong with the brakes other than I’d like the pedal to be a little higher I thought to change them as well as the rotors, bearings, drums, springs, etc, for good maintenance. I bought the stock replacement kit for the front and back from One of the online stores.
Turns out the pads and shoes were fine so it wasn’t necessary but at the same time I’m glad I know exactly what’s new now and while tearing things apart I’m learning more about the car more so thats always fun.
 

7991LXnSHO

wanna catch the space herp
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I lost the lining on a front drum shoes (different car) and did not know it for sure until I went through an underpass and heard the metal on metal of the shoe and drum trashing each other. I did not get a core charge back on those shoes.
 

91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
53
North East
I lost the lining on a front drum shoes (different car) and did not know it for sure until I went through an underpass and heard the metal on metal of the shoe and drum trashing each other. I did not get a core charge back on those shoes.
Damn! It’s the things that are broken that you don’t know about and then find by chance thats spooky, right.... kinda like a house. Everything looks fine but who knows what’s dripping or arching behind those walls that you can’t know about until you stumble upon it and hopefully before a major issue.
 

90sickfox

Wasn't a pretty sight...and I've got big hands
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If there was a fluid leak the peddle would go to the floor....or be soft. If I hadn't seen it happen before I wouldn't have mentioned it. The fluid moves back and forth in the lines. You can get a clog or piece of debri that will stop the fluid from going through in only one direction....like a check ball. A small amount will go past before it stops going through. When you let off it releases that little bit of pressure in the opposite direction.

The prop valve is just an introduced restriction.

More than likely it's a seized up wheel cylinder. I've never had a master cylinder go bad that didn't have a soft peddle. I've been working on 5 to 10 cars a day...5 days a week for 23 years. ( not saying it's impossible...but.... )

A prop valve can cause this issue. It's like a metering valve. It's a common issue on some trucks. They adjust according to the load in the truck. Makes sure there's enough pressure to stop a big load and not too much pressure when empty. The mustang prop valve is non adjustable but if something was clogging the rear port it would act similarly.

I'm not making any 100 dollars bets today though.
 
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7991LXnSHO

wanna catch the space herp
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If there was a fluid leak the peddle would go to the floor....or be soft. If I hadn't seen it happen before I wouldn't have mentioned it. The fluid moves back and forth in the lines. You can get a clog or piece of debri that will stop the fluid from going through in only one direction....like a check ball. A small amount will go past before it stops going through. When you let off it releases that little bit of pressure in the opposite direction.

The prop valve is just an introduced restriction.

More than likely it's a seized up wheel cylinder. I've never had a master cylinder go bad that didn't have a soft peddle. I've been working on 5 to 10 cars a day...5 days a week for 23 years. ( not saying it's impossible...but.... )

A prop valve can cause this issue. It's like a metering valve. It's a common issue on some trucks. They adjust according to the load in the truck. Makes sure there's enough pressure to stop a big load and not too much pressure when empty. The mustang prop valve is non adjustable but if something was clogging the rear port it would act similarly.

I'm not making any 100 dollars bets today though.
When a wheel cylinder or caliper gets stuck, it’s stuck. Why would it stop the wheels and let it move again if the pistons are not moving?
This is why I encourage the pedal test in post #5.
 
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91AOD5.0LX

I have a slight sag in my rear
Feb 20, 2021
253
128
53
North East
I will add:
pump the brake peddle a couple times then put it in gear, holding pressure will the wheels start turning again? Can you feel the peddle move?
So I got around to looking at this more in depth today and doing the Generals test. Bottom line is I’m thinking I may be ok now.
So I started the car with the rear up and pumped the brakes a few times then put it in gear and held pressure. The pedal didn’t move. It stayed steady. The wheels stopped then started to move super slow about an inch in rotation then stopped. I kept giving it gas then hitting the brakes. It seems to stop now and sometimes moves again real slow but then stops when i press a lot harder. The pedal never gets mushy.
I should add that before i did the generals test I drove her around for a long time (first long test drive since doing the new brakes) and did lots of braking forward and reverse. I came home and adjusted the rear brakes again making them tighter on the drum. As I was driving the pedal always felt fine. When I came home Thats when I did the test and although it did move very very slightly at times, it definetly stopped 100% better than the other day. Now the tires stop 100% and either remain stopped or move very very very slow and only about an inch or so then stops again. The other day it would pick up speed and spin even with the pedal down and not stop.
although it’s 99.9% better now that 1% is still something wrong I guess. However I feel that taking it out today for a long test drive since I changed the brakes and using the brakes a whole lot then readjusting them when I came home made a big difference. When I first did the brakes I just took her around the block to make sure they worked but didn’t do much driving. Maybe the shoes, springs, etc, had to kinda move around in position and break themselves in a bit then get readjusted to fix this? Not sure if that makes sense. The brakes and pedal actually feel fine now. Nothing mushy or when I press it down it doesn’t continue to push down. It remains in place.
I feel now that I didn’t have it adjusted enough from the start. Interesting also is that prior to doing the brake job I would feel her pushing forward when I was stopped at a light or in traffic. I would put it in neutral to relieve the pushing pressure. Now it doesn’t do that. She stays in place with no feeling of pushing forward anymore.
In regards to the generals test though, when I pump the brake and put it in gear the brakes hold now and I don’t feel the pedal going down but I also noticed that this is after doing this test maybe twice. The first time I do it the wheels turn slow and slightly then stop again but then when I pump the brakes and do the test a second time in a row then it holds tight. Kinda like pressure building up in the system after using the brakes a few Times and then they work much better.
Hope I’m explaining this all correctly it can be hard articulating sometimes in texts but I really appreciate everyone’s ideas and suggestions. The wheel cylinder(s) and/or master cylinder may still be less than 100% being that they are 30 years old and should be replaced regardless but for now I think I’m confident that the brakes are working properly and that they are much better than they were before i changed them and also better than the other day when I was asking for this advice. The readjustments and break in period seemed to make a difference.
Feel free to drop some thoughts if I’m giving myself a sense of false security or not.
thanks again to all!
 

7991LXnSHO

wanna catch the space herp
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So I got around to looking at this more in depth today and doing the Generals test. Bottom line is I’m thinking I may be ok now.
So I started the car with the rear up and pumped the brakes a few times then put it in gear and held pressure. The pedal didn’t move. It stayed steady. The wheels stopped then started to move super slow about an inch in rotation then stopped. I kept giving it gas then hitting the brakes. It seems to stop now and sometimes moves again real slow but then stops when i press a lot harder. The pedal never gets mushy.
I should add that before i did the generals test I drove her around for a long time (first long test drive since doing the new brakes) and did lots of braking forward and reverse. I came home and adjusted the rear brakes again making them tighter on the drum. As I was driving the pedal always felt fine. When I came home Thats when I did the test and although it did move very very slightly at times, it definetly stopped 100% better than the other day. Now the tires stop 100% and either remain stopped or move very very very slow and only about an inch or so then stops again. The other day it would pick up speed and spin even with the pedal down and not stop.
although it’s 99.9% better now that 1% is still something wrong I guess. However I feel that taking it out today for a long test drive since I changed the brakes and using the brakes a whole lot then readjusting them when I came home made a big difference. When I first did the brakes I just took her around the block to make sure they worked but didn’t do much driving. Maybe the shoes, springs, etc, had to kinda move around in position and break themselves in a bit then get readjusted to fix this? Not sure if that makes sense. The brakes and pedal actually feel fine now. Nothing mushy or when I press it down it doesn’t continue to push down. It remains in place.
I feel now that I didn’t have it adjusted enough from the start. Interesting also is that prior to doing the brake job I would feel her pushing forward when I was stopped at a light or in traffic. I would put it in neutral to relieve the pushing pressure. Now it doesn’t do that. She stays in place with no feeling of pushing forward anymore.
In regards to the generals test though, when I pump the brake and put it in gear the brakes hold now and I don’t feel the pedal going down but I also noticed that this is after doing this test maybe twice. The first time I do it the wheels turn slow and slightly then stop again but then when I pump the brakes and do the test a second time in a row then it holds tight. Kinda like pressure building up in the system after using the brakes a few Times and then they work much better.
Hope I’m explaining this all correctly it can be hard articulating sometimes in texts but I really appreciate everyone’s ideas and suggestions. The wheel cylinder(s) and/or master cylinder may still be less than 100% being that they are 30 years old and should be replaced regardless but for now I think I’m confident that the brakes are working properly and that they are much better than they were before i changed them and also better than the other day when I was asking for this advice. The readjustments and break in period seemed to make a difference.
Feel free to drop some thoughts if I’m giving myself a sense of false security or not.
thanks again to all!
If you can pump them up to work differently, there is a problem.