70 rear brakes locking

[smh00n 87]

This is driving me a bit nuts. I'll try to give you all the issues so you can advise. I have got the brakes working pretty well, but is still locking 1 rear then the other, under harder braking. Fronts lock fine when banged on but the rears will grab on light applications. The right side locks first then left. This is an improvement over both locking at once.

To give you an idea of when they lock, you're driving down the road at 30mph and some goose stops in front of you, meaning you have to hit the brakes not hard-god-damn-I'm-dead but enough to pull up quick. This is when I have rear brakes lock. 90% of the time it's fine but do you want to be in front of me when I use that last 10%?

I have a 70 which I think was a drum brake, non boosted car. When I bought it, it had been converted to front discs with a booster.

What I have done and found so far:

69 pedal and booster have been fitted. I have checked measurements and definitely a 69. Unsure if the pedal box is a 70; will this cause issues? Assuming not, then the pedal and booster match as I replaced them. The booster was new from a US ebay site, it does look the same as what came off. It is a dual diaphragm one and is the 69 with the drop down eye on the booster to pedal rod.

Repro disc master cylinder fitted. The booster and master have been setup by a local brake shop with correct push rod depth. Braking seems normal. Pedal may be a tad soft but.

The front calipers and rotors are probably from an Aussie Falcon. Very similar to the OEM calipers.

Front brakes are not the issue; I can modulate their performance fine.

Distribution block and piping replaced with a later, although similar, distribution block

Standard 302 with about 80 horsepower  :(  C4 auto robbing a further 20 horses

Bled multiple times via the up/down pedal method.

Pedal feels OK, firm, not too much movement before brakes engage

New brake cables

New rear std wheel cylinders

New rear shoes, radius ground to match the drums

New fitting kit for the rear shoes

Rear brakes adjusted by locking drums then pulling back so there is a little drag. I can spin the wheel but there is drag there.

So, am I missing a residual valve somewhere? My workshop manual does not show anything like that But I don't know why a virtually stock brake car has these issues.

When I got the car the brakes were total junk and locked up at will. All the changes I have made have improved it but I am now chasing this last issue. I am a perfectionist if you were wondering...

So, my questions are:

1. Do I need to get the bolt to stop the distribution valve moving when bleeding? I have not done this so far.

1a. Will bleeding them with a pressure bleeder make a difference? I'll probably have to do this anyway as I am solo worker.

2. Does it need a proportioning valve? I see Cobranda has the block and valve but only up to 69. What changed in 70 to remove it?

[stangs-R-me 120]

1969 power disc brake cars had a separate rear proportioning valve and 1970 had it integral to the distribution block.

If you do not have one in your system, that is indeed your problem.   I'd get an adjustable one.  

I converted my car to PDB in the mid 80's using disc brake assy's from a '70 Mustang and  brake booster / distribution block & proportioning valve assy. came out of a '69 Mustang.   Rears are the big 2.5x10" station wagon drums that Shelby put on '65 GT350's.  

I also had lots of problems with the rears locking up.   I then swapped out the stock prop-valve with an adjustable Kelsey/Hayes and it did not matter where I had the prop-valve adjusted they would still lock up.

I finally fixed the problem in 2011.   The original seals on the piston in the distribution block were shot and the one that divided the two sides of the proportioning valve was split, so the prop-valve basically did nothing.   Got the repair (seal) kit for the distribution block from Muscle Car Research (since 2011, the complete distribution block with the proportioning valve became available as a reproduction so this is another option).  

I can now adjust the prop-valve for perfect brake balance.   In my rework, I put a NOS 70's vintage Ford replacement proportioning valve in series with the adjustable one (which was probably overkill), but I wanted to make sure that I got the job done and since I can adjust the K/H to make the rears want to lock up, it certainly is not too much. 

Doug      

[barnett468 418]

If you determine that everything in your system is working properly and the problem still exists and the diameter of the piston in your front calipers is smaller than the original ones, you may need to install rear cylinders that have a slightly smaller diameter as well.

As stangs r me mentioned, look to see if you have the original proportioning valve.

For now, I would adjust the rear brakes so there is zero drag. If there is any drag at all it can cause the rears to overheat then lock up like they are doing now. I have encountered this exact problem a few times before.

An adjustable proportioning valve by itself will rarely adjust the brakes enough to get a good balance because they are adjustable over a very small range only. Also, they work nothing like an original valve and should never be used in place of an original valve but can be used in conjunction with one like stangs r me did.

I would also be hesitant to trust that the shop adjusted the push rod properly on your power brakes and would consider checking it if you can't fix the problem otherwise, but it's a bit of a pita to do and doesn't sound to me like that is the problem.

 

 

 

[smh00n 87]

Thanks guys.

My confusion is with the distribution block; I have bought one the same as the one in the top picture, but without the valve and piping. It takes the fluid from the 2 outlets on the master and splits it 2 ways to the front and one to the rears. It has the brake light switch in the body, so I assume this is also a proportioning valve.

Barnett, the calipers are same diameter as the OEM Mustang ones and the fronts have never been an issue. With the booster, the feel is as good as you can expect so I don't think that's the issue. My problem is only in the rear. Maybe I need to try a doctors forum???

I didn't mention, the rear brakes are 10" x 1.75". I went to the smallest cylinders I could find (7/8"). Next step down is 5/8" but that's a $200 investment. I think I have the standard 13/16"? - 29/32"? in there now so could try the 7/8" again. I'll check today. Barnett the rears lock from dead cold and I've not got them hot.

Another test I did was to put on a local master cylinder (1" bore, inbuilt proportioning valve) with an adjustable valve on but that did very little to change the rear. (I have tried a lot of things to fix this).

Doug, is the prop valve you mention the same as pictured? I have the one without the valve and piping.

I have read that you need to lock the shuttle piston centred when bleeding the brakes. The Ford manual does not mention that so do I need to lock the shuttle piston off when bleeding?

Today I will back off the brakes so no drag, check the cylinder sizing, pressure bleed the brakes and try it out. Watch this space.

[barnett468 418]

yes, you must have a prop valve otherwise your rear brakes will do exactly what they are doing but it might be in the unit you have.

 

[smh00n 87]

The second valve in the pictures is listed as 68-69 only. 70 have the style at the top.

Are you saying the 68-69 with the angled valve piped in is what I need??

[barnett468 418]

The unit you have should have a prop valve under the big nit on the end.

yes you should remove the switch for the shuttle valve abd make a special bolt with a small tip on the end to use as a stopper to hold the valve while bleeding.

 

this is a 1970 ford valve . disregard the word "Remove".

 

[smh00n 87]

righto off the the shed I go

[Rsanter 152]

Check that the disc brake cars don’t use different wheel cylinder.

i know in the 65-6 models if you have a disc brake car the rear wheel cylinder are a different size. The prop valve will be calibrated to that designed balance.

 

[fvike 173]

NPD sells the Brake pressure differential valve switch lock tool

[stangs-R-me 120]

On ‎8‎/‎11‎/‎2018 at 5:57 PM, smh00n said:

 

My set up is using the '68-69 style (lower pic) and instead of the round prop-valve I'm using a Ford replacement that looks like this ...

https://www.ebay.com/itm/68-70-Ford-Full-Size-Factory-Disc-Brake-Rear-Proportioning-Hold-Off-Valve/190782903505?fits=Make%3AFord&hash=item2c6b8c14d1%3Ag%3AW0wAAOSwMXdaoY-p%3Asc%3AShippingMethodStandard!54241!US!-1&_pgn=2&_sacat=0&_nkw=ford+brake+proportioning+valve&_from=R40&rt=nc

… along with a K/H (Ford Motorsport M-2328-A) adjustable one "in series" with the fixed one.   If you want to do a fixed & adjustable in series like I did, it is probably better to do it with the '69 style dist. block.   You could do it with the '70 style block, however only the built-in prop-valve will be bypassed if your front brakes fail and the piston shifts forward.   Where with the '69 style block, both prop-valves are bypassed with a front brake failure.

Not the best picture (should have snapped a pic while the master & booster were still out of the car) but here's a pic of my set up ...K/H on left and fixed one on right below the clutch pedal rod: 

[smh00n 87]

Thanks Stangs. I have found one part of the issue - my 6-month old wheel cylinders have rust through them. The old fluid must have had water in it. cars barely been driven whilst I sort this brake stuff out.

I've ordered new ones and will flush the system completely to clean it.

In Aus, the correct fix it to use smaller cylinders, to take pressure out of the system. I am using 13/16" or 20.64mm wheel cylinders. I think ideally 5/8" ones would fix it, or 3/4" at a pinch. Anyone know of a car that has cylinders in this size? To sleeve them is pretty dear here - about US$150 a pair. Otherwise that valve might do. That will make the 3rd set-up for the car :(

Dumb question #2; how do you get the prop valve piston centred? I made up my own bolt but wondering how you ensure the valve is centred. Bit hard to see down there as well.

[smh00n 87]

Hold the phone! Made a call to a decent brake shop, lovely lady looks up her catalogue and says I'll have  2 x 3/4" cylinders here tomorrow. So I'll give them a go and see how we fare.

[barnett468 418]

if you have brake fluid on your shoes, that can also cause them to lock up.

[barnett468 418]

57 minutes ago, smh00n said:

 

 

[smh00n 87]

No fluid here Barney, not my first rodeo

[stangs-R-me 120]

9 hours ago, smh00n said:

Dumb question #2; how do you get the prop valve piston centered? I made up my own bolt but wondering how you ensure the valve is centered. Bit hard to see down there as well.

I'm assuming you mean centering the distribution block piston.   

Best way is with it out of the car on the bench.   On the '68-69 style you can push it from either end to center it with a punch or nail.   On the '70-up style with access on one end only you can push from the one end, but would need to tap it on a bench top to get it to go the other way.

If distribution block is new or newer, then the above instructions are good unless piston is difficult to move in the bore (see below).    

If it is an old block, it should be disassembled, cleaned, and re-assembled with NEW seals ... also making sure piston moves freely in the bore before re-installing.

Doug    

[Ridge Runner 1,112]

You can center the pin in the car using the brake pedal and opening the bleeders at one end of the car ,it takes two people ,one to gently push the pedal and one to watch in the valve to see the center of the pin as the pin moves. You view the pin through the brake light switch hole with the switch removed

[smh00n 87]

Thanks. Looks like its on the bench, as I have the rear cylinders off. At least I can check the little bludger is working as it should anyway.

Barnettt - my reply was a bit abrupt. I was lucky, the fluid was contained inside the boot so no contamination.

[barnett468 418]

1 hour ago, smh00n said:

Barnettt - my reply was a bit abrupt. I was lucky, the fluid was contained inside the boot so no contamination.

No problem and thanks for mentioning it.

It can be easy for people to forget that unless we know someone, we have very little idea how much they know, so it seems better to offer more info than less info and just let them use what is useful to them out of it. If some of us have to start thinking of what we should and should not post, our muddled brains might melt, lol.

 

 

[DocWok 30]

You sound like your an experienced sort of guy, so at the risk of stating the obvious and not wishing to insult your competency, do you have your brake shoes facing the right direction?

The shorter  leading shoe lining should face towards the front of your car and the longer trailing shoe lining should be facing the rear.

Just a thought.

 

[barnett468 418]

On 8/13/2018 at 8:26 PM, smh00n said:

 

Dumb question #2; how do you get the prop valve piston centred? I made up my own bolt but wondering how you ensure the valve is centred. Bit hard to see down there as well.

You can use the switch that screws into the valve and a continuity tester.

If the switch has 1 wire going to it, there will be continuity from the car body to the post inside the switch when the piston is centered.

If it has 2 wires, there will be continuity between both posts inside the switch when the piston is centered.

You should remove the switch first and check to see if it is working. There is an O ring on both ends of the piston, therefore, no no brake fluid should leak out. If a lot of brake fluid leaks out, the seals are definitely bad.

 

[smh00n 87]

Doc, yep short shoe to the front.

I'll rip this thing to pieces this weekend and make sure it is all right. 

As for experience I've played with cars for years but this is first Mustang so feeling my way a little bit on the nuances. 

[1969_Mach1 333]

It is quite a mixture of brake parts. Nothing to add to the brake lockup issue.

Regarding the residual check valve mentioned in the original post.  It is basically internal to the master cylinder.  Behind the tube seat in the outlet port to the rear brakes.  It's purpose is to help prevent the rear wheel cylinders from leaking by maintaining a very small amount of pressure against the seals.

[smh00n 87]

5 hours ago, 1969_Mach1 said:

Regarding the residual check valve mentioned in the original post.  It is basically internal to the master cylinder.  Behind the tube seat in the outlet port to the rear brakes.  It's purpose is to help prevent the rear wheel cylinders from leaking by maintaining a very small amount of pressure against the seals.

I wonder if my repro master cylinder has that. From memory it is a Dorman (I think the box was green). The fluid in the rear cylinders I found may be because this cylinder does not have one. Might have to try a pressure gauge to see what pressure they have to the rear. I suppose a 2Lb aftermarket valve would be correct?