Need Help with braking problem

[S412gofast]

So we are already with GT350 #1341 for our 1st big show this Sat the 3rd and we are having problems with the rear brakes.

The entire system is new
Dewy restored booster
Dead nuts master cylinder
Calipers restored by Shelby Parts & Resto
New distribution block
All new lines
NPD wheel cyl.  https://www.npdlink.com/product/wheel-cylinder-brake/139832?backurl=search%2Fproducts%3Fsearch_terms%3DT-2262-2%26top_parent%3D0%26year%3D
All new drum hardware
And we do have an original proportionig valve that was rebuilt.


Tonight I adjusted the rear drums, they would spin freely and lock when brake applied with car off. 
I did this becuase after a short drive on sunday the rear wheels got pretty hot to touch.  So I revisited readjusting brakes tonight.
  We went for about a 2 mile drive.  Initlally brakes felt good and we could coast nicley down a slight hill and car was driving really well.
Towards the end of the drive we got out to switch drivers and checked temps of wheels. The rear drums were warm, nothing you couldnt keep your hand on.  So we started to go down street again and coasted again and we could tell rear brakes were binding. Turned aound went back up street to our garage and as approaching garage we had some rear brake squeel.  Into garage, car off, up on jack and I could not turn wheels by hand.

I was able to back off the adjusters to free up wheels, however when doing that the emegency brake would not lock the right rear.

If the proportioning valve was not rebuild correctly could this be the result?

Prior to the adjusting I pulled drums and all looked good inside primary and secondary shoes correctly installed along w springs, equalizer bar and emergency brake components.

[s2ms]

My 66 had exactly those same symptoms after I rebuilt my proportioning valve.  When I pulled it apart the valve body looked good but IMO there must have been some unnoticeable scoring that caused the problem, probably because it wasn't reassembled with everything in the exact same positions as when it was removed, just a guess. After that I decided to only have a proportioning valve rebuilt by a professional...

[Kent]

Normally I also would say proportioning valve if it don't release the applied brake pressure back to the master cylinder. Then the fluid heat up while driving and the brakes start to close more. I would check this with taking out the valve. If this will not work check for any dirt, metal etc. in the brake lines its not uncommon that a little piece can cause this in the right position.

Did you checked if there is a change in the brake pedal feel when this happens?

I also had problems with the wheel cylinders that they were crap out of the box or sometimes the wrong diameter.

[Bob Gaines]

NPD wheel cylinders are typically 3/4 for regular Mustang and not 29/32 like were used for the 2 1/2. However I don't think that is your problem. If you brake shoe and hardware is installed correctly then the proportioning valve is the likely culprit IMO.

[S412gofast]

Thanks all!

I am going to get a repop valve fedexed to change this out tomorrow and revisit another rebuild on the original valve in the future.


Carl

[Skidado]

I had exactly the same issue on #2751. It turned out to be due to warped drums.

[Bossssssss]

Went through this last year with 1432. I could not figure it out initially. Hunted down and changed rear wheel cylinders with different diameter ones. No change. Sent booster back to Booster Steve to have him check it. No issue with it. 

Proportional valve shuttle slide to one side (and I did not know it) when I initially bled the brakes.

When bleeding the brakes in a dry or even semi dry line car (I had changed only my front to back line) what can happen and did happen to me is the shuttle inside the prop valve slid to one side (due to the pressure differential).

Always use this tool that locks the shuttle in the middle until after you get fluid in the entire system: https://musclecarresearch.com/brake-valve-tool

Oh, and make sure the 'brake' light bulb in the dash is hooked up and is not burned out (like mine was). If mine had not been burned out, it would have been more obvious and saved me a lot of headaches.

Lastly, make sure to remove tool and hook your plunger/wire back up before driving the car.

[Bob Gaines]

For closure in this thread I wanted to mention that I saw where Carl indicated in another thread that the replacement proportioning valve he bought and replaced solved his problem.

[S412gofast]

The repop proportioning valve worked.  I locked the piston in the distribution block prior to removing the original rebuilt proportioning valve. Replaced the valve and bled the rear wheel cylinders.  I actually had myself scratching my head for a second at one point as no fluid was coming out of the bleeder.  Yup, I starved the rear reservoir in the master only took about 3 good bleeds and dry!  Filled it up, bled off the master and then the rears.  All good now!

Bob, we were closing out this thread at the same time. 


Carl