Weird rear rear brake problem

[jgkurz 52]

Hi all, I recently replaced my brake master cylinder, vacuum booster, all lines, and the proportioning valve on my 69 Mach1. I bled all corners until fluid came out clear and without bubbles. I started with the passenger rear then driver rear then passenger front then finally the driver front. I repeated the process twice to be sure. The pedal is now hard and firmly locks up the front disc brakes when rotating the tires by hand. All parts are stock type with no aftermarket upgrades. 

The weird part is that the rear brakes will not engage even when the brake pedal is pushed HARD. I can rotate the tires freely. I have bled the rear brakes so clear fluid squirts out which makes me believe my proportioning valve is working properly. The car braked fine before the rebuild so I always assumed, maybe incorrectly, that the rear brakes were working. I find it hard to believe both wheel cylinders are frozen at the same time. The rear brake shoes seem to be installed and adjusted properly.

I'm stumped...   Any suggestions?

 

-John

 

 

[fvike 173]

Did you lock out the piston in the distribution block when you bled the rear brakes? (Remove the brake light switch and use something to hold the piston in place)

[jgkurz 52]

2 hours ago, fvike said:

Did you lock out the piston in the distribution block when you bled the rear brakes? (Remove the brake light switch and use something to hold the piston in place)

Yes, I used a tool from West Coast Classic Cougar to lock the distribution block.

http://www2.cougarpartscatalog.com/store/p/15663-Brake-Distribution-Switch-Tool-New-1967-1973-Mercury-Cougar-/-1967-1973-Ford-Mustang.html

I also bench bled the master cylinder before bleeding all the brakes. 

[jgkurz 52]

I decided to install new wheel cylinders as the next step. They are easy enough to change and I already have the car up in the air so it should be quick. I can't think of what else could be the problem. The cost for each wheel cylinder was only $8.99. 

[Brian Conway 264]

Your on the right track.  To bad you can't get someone to step on the brake pedal.  That way you could just see what's going on ?  Brian

[jgkurz 52]

Thanks Brian. I'll definitely have my wife press on the break pedal before I start ripping parts out.  It would be good to know what's really going on.   -John

[Ridge Runner 1,113]

Care full ,if you step on the pedal with the drums off it will blow the pucks out of the cylinders 

[jgkurz 52]

Good reminder Ridge Runner! 

 

[1969_Mach1 333]

3 hours ago, Ridge Runner said:

Care full ,if you step on the pedal with the drums off it will blow the pucks out of the cylinders 

Yeah, I wouldn't try that.  Will the emergency brake apply the rear brakes?  If so, the only thing remaining are the wheel cylinders.

[Caseyrhe 650]

How old is the hose at the rear splitter block. I had a similar issue on my 68. Would let enough fluid to bleed the lines (gravity) even had a hard petal, but wouldn't let enough get through to operate the cylinders. 

[jgkurz 52]

3 minutes ago, Caseyrhe said:

How old is the hose at the rear splitter block. I had a similar issue on my 68. Would let enough fluid to bleed the lines (gravity) even had a hard petal, but wouldn't let enough get through to operate the cylinders. 

Interesting. All lines and the rear splitter are brand new. The actual brakes @ each wheel are the existing parts including wheel cylinders. I sure hope my new lines and splitter aren't the issue. They would have to be a serious blockage or kink in a line to keep the rear brakes from actuating. Visually the the new lines and parts look fine. Both rear brakes act the same so if there is an issue it would have to be at the block or in the line to the block. 

[Caseyrhe 650]

Overtime the rubber deteriates inside, so if it's new, prob not the issue. FYI, they sell the hose at AutoZone and oreily much cheaper than CJ's

[JayEstes 172]

So, we see 3 potential sources:

1. Prop valve spring piston
2. wheel cylinders
3. rear brake hose from hardline to splitter block

I can confirm I had the same issue on #3 as Casey.  I could push a little fluid thru, but getting pressure thru it kind of wound up blocking it up.  In my case, the hose looked fine from the outside, and I could get some fluid thru, but I think the inner liner was breaking up, and as the pressure increased, it would inexplicably block the fluid path.  Very weird, but possible IMO - since your hoses are new - it's not likely you have this problem.

#2 is easy to check.  If these are new you should be able to push the pucks in and out by hand when the system is open.

But, in my case, I had induced a VERY hard to diagnose problem in the prop valve (#1).  I had bought a great rebuild kit from musclecarresearch.com, and really rebuilt it well (I thought).  I had done everything per directions except for one thing, I had swapped (or mistakenly identified) two of the seals on the red piston (pushed by spring) and the spring cap seal.    

These two seals are close in size, but not the same, but when swapped, the piston was too tight, so tight that the red piston wouldn't slide properly.  This would limit the amount of pressure going to back, and the brakes would bleed, but not work.  Your problem sounds very similar to me.  I got back to Scott Hasselbeck at at musclecar research and he actually put a note in his instructions to not make my dumb mistake.  I have annotated his picture from his instructions posted here (https://www.musclecarresearch.com/valve-rebuild-kh) with arrows, and a red box around the warning.

Hard to believe anyone could make the same mistake on the prop valve (#1) I did... it requires a complete remove to check it - which is a complete PITA.  If you do remove it, make sure the piston slides easily inside the prop valve body.  generally light finger pressure should move it.

Let us know what you find!

Edited June 29, 2017 by JayEstes
corrections

[jgkurz 52]

The problem is solved. I'm happy but slightly embarrased because it was a simple fix. The wheel cylinders were locked up tight with rust. Looking at the outside surface they looked almost new. Wow, looks can be deceiving. I bought my car in 2013 and never inspected them because the brakes were reasonable for a 60's muscle car. They were actually better than all my friends old cars by a long shot. All this time I was operating on just the front brakes. I even went to the drag strip once and had no issues getting slowed down from a trap speed of 101mph. Now that the rear brakes are working it will obviously brake even better. Thanks for all the help and suggestions. 

[RPM 1,190]

Great news. Assume nothing, and doing the process of elimination usually works. 

[MN69Grande 203]

On 6/28/2017 at 7:02 AM, jgkurz said:

Yes, I used a tool from West Coast Classic Cougar to lock the distribution block.

http://www2.cougarpartscatalog.com/store/p/15663-Brake-Distribution-Switch-Tool-New-1967-1973-Mercury-Cougar-/-1967-1973-Ford-Mustang.html

I also bench bled the master cylinder before bleeding all the brakes. 

So if everyone doesn't mind I'd like to throw my own questions into this thread as they are similar in nature.   I had the front brakes upgraded to discs with a Kelsey Hayes style kit.  Since then the brake pedal is mushy.  The car stops fine, in fact if you panic stop it will put anyone not strapped in into the windshield.  But there is a lot of pedal travel before braking starts to happen.  The breaks have been bled several times making no difference.  Last fall I replaced the rear shoes, cylinders, lines, and hardware kits.  Bled again no change.  I have no idea if the Master was Bench Bled, or if the combination style dist block/prop valve was locked in to position during the original bleedings (it wasn't locked during the last bleeding either).

Anyway when I get time I plan to replace the Master Cylinder again, mostly because the one put in there wasn't painted or coated and it's the only rusty bit in the engine bay.  At which time I'll bench bleed the new one.  My question is with the combination style valve how can I make sure the proportioning valve is in the correct position?  I ordered the tool above to lock it in place but want to make sure the valve is in the right position to begin with.  Also I got my hands on a tool from Baer to measure the brake push rod length to make sure the clearance is correct there while I've got things apart.  Any other suggestions?

Finally I've heard with the KH style brakes you should take them off the brackets to bleed them because of the metal line between the 2 halves of the caliper can trap air and you can get a better angle to get all the air out with them off the bracket.  Can anyone confirm this is the correct way to do this?

[jgkurz 52]

I don't have anything I can add to your question since I've never explored brake options for my car. You would probably get a better response to your question if you started a new thread. JMHO