Rear brake drum performance

[fgross2006]

I opened another thread about shoe size and that I had found 2 slightly different rear shoe sizes listed on multiple websites.

I determined that the 9 x 1.71 was the correct one and replaced my rear drum brakes earlier in the week. They can be tricky so I took my time and bought new spring kits etc. After I got both sides assembled and adjusted I put the tires on and since I had the car on stands I checked the rear brake function and was surprised to find that while engaging the brake pedal both tires continued to spin, even with the pedal pushed as hard as it could get it the tires still spun.

I tightened up on the adjustment some more but honestly if I tightened any more I would have cracked the shoes. I took the car to my mechanic and had him go over the whole set up. The only thing we found wrong visually was the self adjuster cable hook fell out on the driver side. Regardless of that, it would have affected self adjustment, not braking function. So I bought new self adjuster kit fpr the driver side only since the passenger side was assembled OK.

The wheel cylinders appeared to be functioning when stepping on the brake pedal and there was no fluid leaks in either side. We determined there to be poor pressure to the rear so we replaced the master cylinder. Now he had trouble bleeding the rear passenger side. They spent a better part of the day and it kept producing air. We tried putting the disconnected brake line into a large cup of brake fluid and pumped the brakes until bubbles stopped coming up and there was definitely a bled line. Reconnected the line, tried to bleed out the minute amount expected from reconnecting to the wheel cylinder and now the line is filled with air again.

He tells me he suspects the wheel cylinder is sucking air so I say if I'm changing one, I'm changing both. So off with everything and now a pair of brand new wheel cylinders. This time the system blead easy and the pedal is firm and high as it should. Drove the car and has great stopping power. The only thing, the rear tires still spin when the brake is engaged. This cant be normal. The car has great stopping power and doesn't feel as though I'm only using front brakes. I know that feeling when only the fronts are braking. Yet, why is it that engaging the brake while the car is off the ground results in the rear tires slowing but not stopping?

My mechanic suggested I look for a proportioning valve. Has anyone here had this issue or seen of it? Any links to the correct proportioning valve or even an upgrade that allows adjustment of pressure between front and rear?

Also, this whole mess allowed us to find the rear driver side axel seal was leaking oil so I also ended up pulling the diff cover, replacing the driver side seal and dropping 2 fresh quarts of gear oil and a bottle of friction modifier. I just rebuilt the 8.8 rear about 5 or 6 years ago. I'm surprised one of the seal leaked so soon considering the car isn't driven much.

[Bentley]

How old is the soft line from the brake line to the rear housing lines? I would replace it if you don't know how old it is.... These can collapse from the inside..... Check all of your metal brake lines too,, If any look questionable, replace them all.....

[fgross2006]

How old is the soft line from the brake line to the rear housing lines? I would replace it if you don't know how old it is.... These can collapse from the inside..... Check all of your metal brake lines too,, If any look questionable, replace them all.....

the soft lines were replaced about 6 years back when I did my single to dual exhaust conversion. Part of the process was switching the rear brake line setup to match an 86 and up. I'd suspect the steel lines from proportion valve to the rear before the soft lines at the drums. They are not old enough and haven't been used enough.

I'm having no luck finding a proportion valve for an 84 even used. I see a few for 84-86 SVO but their must be a difference.

[1984GTTurboVert]

I'm having no luck finding a proportion valve for an 84 even used. I see a few for 84-86 SVO but their must be a difference.

I had a really hard time finding a proportioning valve for my 84, it was full or crust and the front line was completely plugged. Car had no stopping power at all even standing on the pedal and any force was coming from the overadjusted rear to compensate.
I wound up with an expensive nos one that was mislisted for another vehicle because it was either that or some janky adjustable thing i didnt want to even get into messing with that.

[emerygt350]

When you have them perfectly adjusted they should drag ever so slightly. My rear drums (7.5) work great. When you say they won't stop the wheels is it when the engine is running? If you depress the brakes do the shoes move with the drum off?

[fgross2006]

When you have them perfectly adjusted they should drag ever so slightly. My rear drums (7.5) work great. When you say they won't stop the wheels is it when the engine is running? If you depress the brakes do the shoes move with the drum off?

With the car on a lift or Jack's and the rear wheels off the ground, putting the car in drive and spinning the wheels, applying the brake does not stop the rear wheels from spinning. They are adjusted as tight as possible.

And we checked wheel cylinder function with drums off. They do move when brake is applied. So why do the rear wheels not stop or lock even when the brake is pushed as hard as possible?

I'm thinking adjustable proportion valve may be needed?

[qikgts]

With the car on a lift or Jack's and the rear wheels off the ground, putting the car in drive and spinning the wheels, applying the brake does not stop the rear wheels from spinning.

How much throttle are you giving it in the above example or is this at idle with no throttle input?


If it comes down to needing the prop valve rebuilt, that can be done too.

https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

[Bentley]

How much throttle are you giving it in the above example or is this at idle with no throttle input?


If it comes down to needing the prop valve rebuilt, that can be done too.

https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

Thanks for this lead... I ordered one of these kits for my parts stash....

[qikgts]

Thanks for this lead... I ordered one of these kits for my parts stash....

No sweat!

[fgross2006]

How much throttle are you giving it in the above example or is this at idle with no throttle input?


If it comes down to needing the prop valve rebuilt, that can be done too.

https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

I'm not giving it any throttle. With the tires off the ground and the car in drive, the tires spin on their own. Engine idle speed is the answer to your question. And at minimum RPM, the rear brake at maximum pressure on the brake pedal will not stop the rear wheels from spinning.

The adjustment is tight enough to feel maximum drag on the drum when trying to spin the rear wheel with the trans in neutral. Adjusting them tighter is not the answer. And we replaced both wheel cylinders and verified movement when the brake is applied with the drums off.

[qikgts]

I'm not giving it any throttle. With the tires off the ground and the car in drive, the tires spin on their own. Engine idle speed is the answer to your question. And at minimum RPM, the rear brake at maximum pressure on the brake pedal will not stop the rear wheels from spinning.

You'll have to have someone check the system with a pressure gauge at the rears to confirm if this is some kind of issue with fluid. The correct PSI specs should be in the factory service manual if not, I'd bet Jack Hidley will know.

Maybe the wheel cylinders aren't actually correct for the application and can't move the shoes far enough? Presuming that you have the primary and secondary shoes in the correct positions on both sides?

[fgross2006]

How much throttle are you giving it in the above example or is this at idle with no throttle input?


If it comes down to needing the prop valve rebuilt, that can be done too.

https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

I ordered one. I'd feel better if it came with a new spring but the seller says its not needed and he doesn't have one to furnish.

Honestly though, if I rebuild this and it doesn't solve the issue I'm gonna be pissed. My mechanic prefers trying the rebuild before moviing on to an adjustable proportion valve.

What sucks is he has a junked 87 in his yard that i have picked for parts before but it has a plastic single chamber master cylinder and totally different proportion valve.

[KevinK]

I ordered one. I'd feel better if it came with a new spring but the seller says its not needed and he doesn't have one to furnish.

Honestly though, if I rebuild this and it doesn't solve the issue I'm gonna be pissed. My mechanic prefers trying the rebuild before moviing on to an adjustable proportion valve.

What sucks is he has a junked 87 in his yard that i have picked for parts before but it has a plastic single chamber master cylinder and totally different proportion valve.

You do know the rear brakes are all the same drum brakes. The 87 4 cyl disc brakes are the same as the 84-86 brakes. That means the 87 4 cyl. master cyl should work on the 84-86.

[qikgts]

Honestly though, if I rebuild this and it doesn't solve the issue I'm gonna be pissed.

Did you buy a $10.00 bouns tool for your mechanic when you ordered the kit?
https://www.musclecarresearch.com/brake-valve-tool

[fgross2006]

How much throttle are you giving it in the above example or is this at idle with no throttle input?


If it comes down to needing the prop valve rebuilt, that can be done too.

https://www.musclecarresearch.com/va...-E0ZC-2B328-AA

So I got the o-ring kit and the break bleeding set-screw. I'm hopefully going to start this project this upcoming week depending on my mechanics availability. I read through the instructions on cleaning the hard parts in ammonia. I was thinking white vinegar. Anybody have experience with cleaning the brass parts on a proportion valve?

Also, It says the clean the bores so I will need round wire brushes. Does anyone know offhand what diameter the bores are on a proportion valve and what size wire brushes I'll need to clean them out?

Also, brush by hand or on a power drill rotating at medium speed better?

Last thing, what holds the proportion valve bracket to the frame? is it welded, riveted? I don't see screws or bolts to remove.

[emerygt350]

When it works it works well, autocross yesterday I was noticing when and how the rears would lock. Very progressive, I think it was a well designed system. Where is the proportioning valve?

[gr79]

Locked the rears up driving in the rain Friday.
Car rear slides to the right a couple feet at 40 mph.
Was unintentional. A quick firm stab did it.
Another car made a jerky turn into a median turnaround cut.

Wonder what these rear brakes would do during an off the ground brake check?

474 and 569 shoes are similar enough to interchange most year vehicles using them.
Motorcraft Br75B do 9" Mustang 83-93, Capri 79-86, T-Bird 81-88, LTD/Marquis 83-86.

474 number adds Fairmont/Zepher 78-83. Mustang II, Pinto back to 1975.
474 year span is 1975-1988.
569 number year span is 1986-1994.
BR-75B year span is 1983-1994.

Wheel cyl got changed over in 1980. Difference or change unknown.

[emerygt350]

I forgot, the first time I rebuilt my brakes I screwed up the shoes, that does make a difference.

[qikgts]

Good deal! Guess we'll all find out soon!

Just to complete a thought... I mentioned above that maybe the new wheel cylinders are somehow incorrect and I bring it up again because I noticed that MM has wheel cylinders listed for sale that provide more rear bias. They recommend them for specific drag car combinations with big's and little's. I'm not advising they will fix your issue but instead mention it to make the point below.

https://www.maximummotorsports.com/B...air-P1620.aspx

Obviously, there are parts out there that are real similar to one another and most people wouldn't ever know the difference by looking at them, but would wind up getting bad performance once they were installed. Call it Chicom, spec drift, application consolidation, whatever. Over the years we've all seen the parts listings for our cars become inaccurate and it blows. Maybe you can do some measuring/comparison with known correct parts/the ones that were on there before?

[emerygt350]

Did you do an excellent job bleeding your MC? Brake light is troublesome. The valve shouldn't cause that.

[Bentley]

If you reseal your valve, use only brake fluid on the orings for installation lube.... Do not use grease, oil or wd40....

[qikgts]

I cleaned mine out when I gutted it. I just used clean brake fluid and brake cleaner.

Any nylon or brass bristle brush that fits will work. Small tooth brushes included. Do enough to get it clean but don't go so far that you're removing material as brass is pretty soft. Hand power should be fine for this.

If I remember right there's a stud on the bracket that goes through the sheet metal and it's retained with a nut on the wheel side.

Circling back to the original concern, were the drums ever measured and confirmed to be in-spec? Also, was the master cylinder push rod length checked and verified to be correct?

[fgross2006]

I cleaned mine out when I gutted it. I just used clean brake fluid and brake cleaner.

Any nylon or brass bristle brush that fits will work. Small tooth brushes included. Do enough to get it clean but don't go so far that you're removing material as brass is pretty soft. Hand power should be fine for this.

If I remember right there's a stud on the bracket that goes through the sheet metal and it's retained with a nut on the wheel side.

Circling back to the original concern, were the drums ever measured and confirmed to be in-spec? Also, was the master cylinder push rod length checked and verified to be correct?

The drums were installed new on my last rear brake job and they were smooth enough to not need a cut. As for the master cylinder I dont know. I never thought there was a rear brake problem until I tried hitting the brakes with the rear ties off the ground, so as far as I know this may have been an issue I was not aware of prior because the car does stop on a dime.

[Bentley]

Been thinking about this post....
You are either not getting enough flow or enough pressure it seems to me... Of course this is assuming you have the right shoes and wheel cylinders...
Did this problem start after the new parts??
I think I would disconnect the brake line on the pass side firewall ( at the union nut) then with someone pumping the brakes see how much fluid and pressure you have there.....
If OK there, reattach it and remove the hard brake line where it attaches to the soft line for the rear end...
Again if OK there, remove the soft line from the 2 lines on the housing and check flow and pressure there....
Just my thoughts here.... Something is not right, and I think this might help you figure out WHERE the problem is....

[qikgts]

10-4. I hear ya.