Rear Brakes Locking

[SBTX73351W]

73 Mustang, front disc, rear drum.

BOTH rear brakes are locking at barely a tap of the pedal at very slow speeds. Now, when I say locking, I mean locking as if you slam on your brakes at 35mph in daily traffic to avoid an accident. Only this locking is occurring at speeds UNDER 10mph in my own driveway. I'll tell you what I've done so far to it in the last year, but I still have not been able to clear up this problem.

NOTE: This problem was happening both before AND after all of the below-mentioned work was done...

-Rebuilt rear wheel cylinders
-Adjusted rear brakes manually rather than letting them adjust
themselves
-Replaced master cylinder
-Replaced front brake pads
-Bled entire brake system THREE times
-Jacked car up, rotated tires to check for stopping of the front
brakes. They both stopped when pedal was applied.



If I think of anything else I've tried, I'll post it as I remember it.

It would seem to me that both rear brakes locking up at speeds of less than 10mph at barely a touch would indicate that the front brakes are NOT doing their job, since they do most of the stopping.

The real question is, what am I missing? I know I have adequate fluid in the system, there are no leaks, but something is failing.

The only two things I haven't checked are a) the front brake calipers (because in my lifetime, I've never had to rebuild a set of those on any car I've ever driven daily) and b) the proportioning valve.

*I don't know if that is what it's called, correct me if I'm wrong on that...it's a device that's hooked up in the path just outside of the master cylinder on the driver's side fender well.

If anyone has any ideas, I'm open to them. This brake problem is dangerous, I can't even get a proper response out of the brakes in my own driveway, much less out on the street (where I will NOT take it).

Thanks,

SBTX73351W
73 Grande
351W 4V
C4 Trans.

 

[stangboy68]

I would suspect the proportioning valve. Is it the OEM type or the aftermarket adjustable type? It seems that your brake bias is off. I would replace the pro valve or if its adjustable adjust it better.

 

[Platonic Solid]

73 Mustang, front disc, rear drum.

BOTH rear brakes are locking at barely a tap of the pedal at very slow speeds....

-Adjusted rear brakes manually rather than letting them adjust themselves...

Have you tried just losening the rears a bit and allowing them to adjust themselves?

Are you sure you didn't accidentally switch star wheel adjusters between left and right drums?

Are you confident that the adjuster cables are properly installed and working?

 

[2nd Mustang]

Check and see if you have some grease applied to the backing plate where the metal edges of the shoes rub on the backing plate. Sometimes if no grease is there, the shoes will rub and hang up. Don't get any on the shoes themselves, then you'll have the opposite problem, NO STOPPING.

 

[SBTX73351W]

I would suspect the proportioning valve. Is it the OEM type or the aftermarket adjustable type? It seems that your brake bias is off. I would replace the pro valve or if its adjustable adjust it better.

OEM. I think it's the original valve that came with the car, if you can believe that. I've been told these items are not typical off-the-shelf parts you can find just anywhere. In fact, some people have told me they're junkyard pieces.

 

[SBTX73351W]

Have you tried just losening the rears a bit and allowing them to adjust themselves?

Are you sure you didn't accidentally switch star wheel adjusters between left and right drums?

Are you confident that the adjuster cables are properly installed and working?

Yes, I have loosened them a bit and allowed them to try and adjust themselves. Several dozen times over the last year, in fact.

And to answer your other two questions, previous brakework possibly putting the components on bass ackwards did cross my mind this morning, so I pulled both wheels and drums on the rear. All the cables, star wheels, springs and components are arranged by the book (checked three different Mustang manuals that I own to confirm this). I thought that might actually be what the problem was/is, so I checked three different reference books, then looked over both wheels carefully with the drums pulled this morning. They both look fine, and all components on the back look correctly installed.

 

[Platonic Solid]

NPD sells rebuilt Proportioning Valves - 2B091-20A $89.95

 

[Zooted_plus]

Are you sure that both long shoes and both short shoes aren't paired together? This will cause the rears to lock up, although mainly the side with the two long shoes. There was a 69 390 4v in my shop yesterday with that problem... the guy did the shoes himself... couldn't figure out why his brakes were locking up... he said they were working "too good".

 

[StangPowerSoon]

Another side note you might want to check on, is your Drum springs. Spring kits are relatively inexpensive and can be the cause of many woes if your return springs are shot. Do a simple check on your springs by dropping them about 2 feet onto a concrete floor. If the sprangs make a loud clang, then their shot. If they make a dull thud then your ok

 

[Timmy]

Sounds like the proportioning valve to me. If you still have the original, it might be a good idea to change it out. $89.99 is not that much if it corrects the problem.

Just my 2 cents

 

[pabear89]

Is the light wire still connected to the porp Valve?
If not reconnect it and the light will probably stay on.
It is telling you that there is a malfuntion in the brake system.

It is eather stuck and allowing the pressure to run to the rear brakes and very little to the front.
Or the connection from the master cyl to the valve is backwards.
I.E. Master front brake line connected to the rear valve port.

I would do a complete line connection follow thru and reference the manuel.

 

[68GEETEE]

go to this site, they have the valve you need.

www.mpbrakes.com




p.s. dont buy the adjustable aftermarket valve with the little round knob. . It is only to apply pressure to lock your front brakes, ie for race track applications. get the CORRECT valve at the website I mentioned above.

 

[GaPonyFarm]

p.s. dont buy the adjustable aftermarket valve with the little round knob. . It is only to apply pressure to lock your front brakes, ie for race track applications.

Thats an adjustable proportioning valve, and you can fine tune your rear brakes much better with it, than you can with the OEM type. It has nothing to do with the front brakes, or racing. I recommend it over the OEM type...

 

[bnickel]

it is definitely the prop. valve. there is a check valve in the prop.valve that is supposed to be centered so it meters the fluid correctly, what has happened is that someone most likely did not bleed the brakes correctly on a previous brake job. it is easy enough to correct. first loosen the lines FROM the prop valve TO the brakes just enough that the brake fluid will "seep" out then get a helper to stomp on the pedal a few times while you gently tap the valve with a hammer a few times, this will re-seat the check valve and if done correctly you shouldn't have to bleed the brakes afterwards. next make sure you have a good pedal and take the car for a short test drive and see if you are still having the problem, if you are then get the car up to about 20-25 mph and stomp the brake pedal several times and re-test, repeat this procedure as necessary. once it is functioning properly again go back and bleed the brakes just to make sure all the air is out, and make sure you follow the correct procedure for bleeding them, start at the wheel cylinder furthest away from the master cylinder, in this case the passenger rear, next do the driver rear, then the passenger front and finally the driver front. if this does not correct the problem then you will need to get a new prop.valve, but about 99 times out of 100 this will fix it. good luck and let us know how it turns out for you

 

[68GEETEE]

Thats an adjustable proportioning valve, and you can fine tune your rear brakes much better with it, than you can with the OEM type. It has nothing to do with the front brakes, or racing. I recommend it over the OEM type...

You might want to check with Master Power Brakes and see why they say NOT to use the adjustable valve. I called them to order it and they talked me out of it, and instead sold me the correct oem replacements, which by the way work great.

 

[dan220]

adjustable proportioning valve

i'm trying to install an adjustable proportioning valve but i have lost the instructions on how to give the front disc brakes more pressure. Can anyone help me complete this sentence?
I have to (increase/decrease) the valve in order to give the front more stopping power. SSBC Adjustable Proportioning Valve

Thanks

Disc brakes in the front & drums in the rear. Ford 4 piston brake caliper

 

[Spoofty]

I had the opposite problem with my fronts locking. Turns out it was the rubber brake line going to the axle.

 

[302 coupe]

i'm trying to install an adjustable proportioning valve but i have lost the instructions on how to give the front disc brakes more pressure. Can anyone help me complete this sentence?
I have to (increase/decrease) the valve in order to give the front more stopping power. SSBC Adjustable Proportioning Valve

Thanks

Disc brakes in the front & drums in the rear. Ford 4 piston brake caliper

any decrease in rear pressure creates an increase in front pressure. You cannot just create more pressure, unless you change master cylinders. On my Wilwood valve, I turn the adjuster in (like tightening a screw) to restrict rear pressure.

 

[davishokies]

it is definitely the prop. valve. there is a check valve in the prop.valve that is supposed to be centered so it meters the fluid correctly, what has happened is that someone most likely did not bleed the brakes correctly on a previous brake job.

From my understanding (I'm no expert), the plunger in the prop valve ONLY identifies a failure (loss of pressure) for either the rear or front brake system and doesn't meter or shut off fluid to either side. I thought you have a dual bowl MC for a reason...two seperate and independent systems. Once you fix the failure, the valve needs to be reset, but this step is often ignored.

Another reason I say this is because before I changed to front discs, my brake light remained on...I unplugged it...AND I had both front and rear brakes. There are probably thousands of owners who have just unplugged it get rid of the light.

Are you saying that becuase the light is on, you have either front or rear brakes but not both?

 

[mustangdave]

You can still have both, but not the correct amount. Once the valve moves so far, you will have extreme braking problems as mentioned. Been there, done that. It is easy to get the valve off center when doing a major brake job/bleed.