Ok, here is another disc upgrade problem that I'm hoping someone here can shed some light on.

First the the disc brake swap info:

1968 2 door sedan Valiant w/ 108" wheel base
Stock SBP 1968 7 1/4" rear with newer 1981 F body disc spindles (they DO work just fine on the street!) and 1976 A body upper control arms w small BBP rotors in front.

New Raybestos manual disc/drum M/C with 1 1/32" bore
New prop valve (all in one 5 port valve p/n BLK 250 from inline tube) for all 69-74 drum disc A bodies
Reman'd slider calipers from NOPE-AH! with 2 3/4" phenolic pistons and semi metallic pads
13/16" rear wheel cylinders with new 9" rear drums and semi metallic shoes and springs and adjusters

Problem is:

After about 10-15 miles of normal city driving (45mph-55mph) the rear brakes heat up enough to be HOT to the touch and smell of brake linings but stop fine/normal. The front disc are only warm at this point.

However, when heavy stop and go traffic is encountered (20mph-5mph stop and go gridlock and 95 degree heat), the rear brakes start to drag noticeably even when not pressing the pedal and the front discs feel like they are not grabbing like they should.

This set up worked great with worn out rear drums and old shoes for over a year and a half. I just recently replaced the worn out rear brake drums and shoes with new 9" Raybestos semi metallic replacement shoes and Chinese drums and noticed the problem shortly there after. I do believe that I have the adjusters set correctly as per the FSM but maybe I need to run them looser?

Any ideas as to what is causing this? I think that it may be time to install a rear prop valve to adjust the rear brakes to engage later in the brake system but wondered if I needed different sized rear wheel cylinders (15/16") instead? I dunno, I an pretty miffed at this point.

Anyone have the same experience and found a cure?

Your help is appreciated!

CMS

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'68signet

"Living analog in the digital age."

Sorry, I forgot to include that the drums are Raybestos brand made in China, not no-name Chinese. Not sure it makes a difference anymore but they do go round-and-round without a wobble (for now).

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'68signet

"Living analog in the digital age."

Is brake fluid pressure bleeding off fully after the brakes are applied. In other words, is there a problem with fluid returning to master cylinder once brake peddle is released?

Did you keep the old master cylinder, or replace it with a new one designed for disk / drum system?

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Do you have the leading and trailing shoes in the correct spot?


Or the shoes might just need to wear in to conform to the brake drum diameter......
(my truck did this, they got really hot; then I backed off the adjustment and they were fine)

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I did replace the master cylinder two years ago with a new, not reman Raybestos unit for a manual disc/drum 74 Dart. Also, I checked the shoes and the primary shoe (ones with less brake material) are on the front side of both rear units. The shoes are broken in for sure, they just now in the past week have been acting funky.

After I posted my question I went outside and tore into the rear brakes and found that they were barely dragging when cold. So I adjusted them looser by 10 to 12 notches on the star wheel as per the FSM. Went for a test drive for a mile or two and came back. Kinda acted the same after the third or fourth brake peddle application. Jacked it up again and HOLY COW. Those puppies were tight!

So, I take some time and let it cool off enough to get the drums loose. I open both sides up and try to adjust them looser to make up for everything being so tight still. I ended up backing them off 45 clicks on the adjuster wheels and still was not completely free wheeling.

Then I see that the shoes are not retracting to the proper at rest location. Come to think of it, the cylinders seem to not wanna move much. They retract, kinda but you have to beat on em with your fist! Dang! I think I fried the inner seals of the cylinders!

All of this after rush hour traffic in 95 degree heat for an hour and a half. I prolly did fry the cylinders after that.

I am wondering if the original front to rear brake line is rusted enough inside to cause a blockage or slow bleed off condition that keeps the cylinders pressured up too long.

Still miffed.........

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'68signet

"Living analog in the digital age."

Crack open the bleeder valve and see if they retract.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

You've got a hydraulic problem here. Could be a rear brake flex hose that has degraded internally so it is acting like a 1-way flapper valve, could be a faulty proportioning (combination) valve that's holding pressure to the rear brakes, could be an improper residual pressure valve in the master cylinder (though there shouldn't be one at all).

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Yes S.S.D., I'm beginning to think that is the problem. I'm going to lift the rear and see if the brakes are dragging since yesterday's adjustment without touching the brake peddle.

I'll bet that the 43+ year old rear brake line/hose is the culprit. They are the only parts in the system I have not replaced yet. If not there, I'll be forced to start from the firewall and work to the wheels.

Ahh, always with the unusual problems this car is but boy-howdy! am I learning something along the way.

I have a feeling it's going to get messy real soon before it get better.

Thanks for the advice everyone, I'll keep you posted when something is found.

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'68signet

"Living analog in the digital age."

Are you return springs correct for a 9" drum set up? I believe the 10" drums kit is different with longer springs. If you have the longer springs, the shoes won't retract completely. If there is too much drag, the "servo" action will self wedge the linings, cooking the brakes.

OR; as Dan said, Hydraulic problem.

OK,

I checked the shoe return springs and they are all the same size that were previously in place. I also cracked open a bleeder valve and some fluid did come out but not very fast. I do get free movement in each of the rear wheel cylinder pistons from side to side so I do not think I fried the inner wheel cylinder seals.

I visually checked in the master cylinder reservoir to see if anything was up since I moved the car recently. Each port in the bottom of the pots are clear and I can see through them to the shiny piston inside the M/C bore. The rear brake pot fluid is a little more dirty than the front but not much. The brake peddle push rod is the same one from the old drum/drum system but it has plenty of room to return the M/C piston to its fully extended position (at rest).

And lastly, the emergency brake cable is fairly slacked/loose and neither emergency brake lever is engaged on either secondary shoe.

The more I nose around the more I think there is some sort of obstruction in the front to rear brake line. New parts will be here next week to change the front to rear hard/soft lines out so we shall see........

Thank you all for the suggestions and ideas. What an education!

More to come.......

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'68signet

"Living analog in the digital age."

Everyone has given you sound advice on the hydraulics.

Did you check the "toe-heel clearance" on your shoes & drums? On any Bendix style (self energizing) drum brake, you NEED to have the radius of the shoes to be slightly smaller than the radius of the drum.

I usually check this by taking the shoe (off the car), holding it in the drum. I should see a slight gap at the top & bottom of the lining, while the middle should press against the drum. I should be able to "wiggle" (twist) the shoes some. If there's a gap in the middle between the drum & lining, or no gap at the ends, (you can't rock the shoe at all) then you have a full-fit condition. That's make the brakes very grabby & hard to modulate.

In the old days you used a radius grinder to remove lining material in order to get the correct drum fit (asbetos what?). Now a-days I would get different shoes (& hope they fit better), use a sanding board to dress the shoes (since asbestos is "almost" never found in new shoes), or possibly have the drums cut a little bit larger.

I think whatever your problem is that the semi met shoes might make it a bit worse- I'd imagine they would expand more when hot compared to normal shoes.

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63 Valiant Wagon
225 - 4 bbl

The brake shoe linings are somewhat smaller than the inner brake drum radius and do not make full contact so I believe they are the correct shape. However, I do suspect the semi metallic material is causing more friction than needed.

I re-checked the rear cylinders movement with the shoes off last night and they are sluggish at best so yes, they are fried. So today, I bought new cylinders (13/16") and new non metallic shoes and once the new rear lines get here I will put it all back together and then function check. If the brakes still don't retract after that then I definitely have a prop valve or M/C issue.

Again, thank you all for the suggestions! This is a great forum, I'm going to make another donation!

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'68signet

"Living analog in the digital age."

Well, finally got to the Valiant this week. Installed the new front to rear line and rear flexible hose. Then I rebuilt the wheel cylinders and put new bonded shoes on too. Checked all the springs and their locations as well as the adjusters (I think I may have read the chapter on 9" brakes about 50 times!).

Put it all back together today and after a spin around the block it still keeps the rear brakes on. So, I cracked a rear bleeder and ....Voila! the brakes retract to the position of rest. You know what that means!

I checked the travel of the master cylinder push-rod and it is returning fully to the stopped position on the back of the casting. I also pumped the brakes back up and loosened the line coming out of the rear pot of the master cylinder. No excessive pressure there.

I suspect the problem is in the aftermarket Inline Tube proportioning valve but of course I ran out of time today.

At least I'm getting closer....I think.

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'68signet

"Living analog in the digital age."

Here's a friendly suggestion that may or may not correct the problem but, it will not cost you anything. Double check with the parts store to make sure you really have a disc/drum master cylinder and not a disc/disc mc. Sometimes the wrong part gets put in the box at the manufacturer. I have been in manufacturing for many years and I can tell you it happens, even in the best of companies. I would also verify that the proportioning valve does not have a residual pressure check valve in place for a disc/disc setup.

If the old rear shoe were well worn, organic linings, a residual pressure valve in the system may not have had much effect on the old rear brakes.

Good luck, and let us know what you find

You've got it backwards. Drum brake systems (prior to 1973) have a residual pressure valve. Disc brake systems do not have a residual pressure valve.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.