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9/07/2017 7:39 AM  #1


MS GT Brake swap, Master Brake Cylinder defective ?

Hi All,

After reading almost every topic i could find here on the board, on the world wide web and after numerous attempts to bleed my new brake system in every possible way , I still don't have enough pressure on the system.
Recently the brake booster was replaced for a remanufactured OEM one, this changed absolutely nothing.
I slowly start to suspect the master cylinder although it is new and was never used before.

If I push the brakepedal with engine off, the pedal will go down 2/3th of the way, rear brake line pressure is around 500 Psi.
If I push the brake pedal with engine running, it goes 2/3th of the way down easily and if you press it a little harder, it feels as if you press it against a stop or against something hard and solid, rear brake line pressure stays at around 500 Psi but rises a small bit more if you really stand on the pedal.
The brake pedal doesn't pump up if you press it a couple times and drops a little when the engine starts like it should.
Both the M/C and Booster are never used before, Vacuum booster check valve is OK and attached straight to a vacuum tower in a manifold runner with noting else attached to it.  If you drive the car, it does brake but in a terrible and absolute insufficient way.

I' have swapped my 1970 Mustang 4 corner drum brake system years ago to a Granada front disc swap and now to a complete Mustang Steve 2008GT disc brake swap set up, front and rear.
Before the Swap it braked really good !

What's installed...
-Front 13,2" GT Rotors (New) with 2011-2012 Mustang GT brake calipers (New-Roush Take off's)
-Rear 2005-2013 GT-V6 OEM rear calipers and rotors (new)
-9" Brake Booster New (Reman. Cardone)
-Master Cylinder, aluminum 1" bore, New (Bought @ Mustang Steve in May 2014, never used)
-Original 1970 Drum Brake distribution block (also see topic-->) http://fyi.boardhost.com/viewtopic.php?id=6342
-New Ni-Cop Brake lines
-New Flex brake lines from MC to distribution block and from brake lines to calipers.
-New adjustable proportion valve with gauge, mounted in the rear brake line on the tunnel.
-New EBC brake pads.
-Hand brake attached, Push rod between M/C and booster adjusted, no brake lines above M/C level.
-Bleeder screws all on top of callipers.
-M/C bench bleed. 

I did notice (when adjusting the Pushrod) there is a small amount of fluid on the outside of the M/C


There seems to be a tiny bit of pressure left in the rear brakes when brakes are not applied.



Brake distribution block is mounted 1" lower to prevent brake lines to be above M/C level.




Is there anything that I might have missed ?
I'd hate to order and ship a new Master brake cylinder again to find out later it's caused by something else that I've missed.

Thanks for any help !

Mich.

Last edited by Gearz (9/07/2017 7:55 AM)

 

9/07/2017 9:32 AM  #2


Re: MS GT Brake swap, Master Brake Cylinder defective ?

Man your problem sounds sort of like what I had.  With 12.5" GT fronts, Explorer rears, that same MC and a gutted disc-disc prop. valve I was getting about 750 psi to front and rear.  I checked manifold vac and only had 9.5" (6000 feet elev.) due to the cam, apparently.  Added vac pump and canister and now it works like it should.
One thing you don't mention is the manifold vacuum.  You must have 15-20 inches of vac for that booster to work properly.  That said, 500 does seem awfully low even with no booster.  Also, you only mention rear pressure.  What is the front pressure?  If the front is good you also could have a problem with that adjustable proportioning valve.

And...I just looked at your pix again and I'm wondering if your "1970 distribution block" is also a prop. valve and causing trouble.  Or, could the slide valve in that thing be off center.  Also it looks like the flex lines from the MC to the original block make a loop that's higher than the MC piston which could cause bleeding trouble.

BB

Last edited by Bullet Bob (9/07/2017 9:41 AM)


"you get what you pay for, good work isn't cheap, and there are NO free lunches...PERIOD!"
 

9/07/2017 12:02 PM  #3


Re: MS GT Brake swap, Master Brake Cylinder defective ?

500 psig is about what I would expect with a 1.0" unboosted Master Cylinder. That is enough pressure for drum brakes. Your four wheel disc brakes need about 1200 psig. You said the brake pedal "just stops", which tells us that the MC is not leaking. A leaking MC, either internally or externally, will result in your foot slowly sinking to the floor when you are applying the brakes. It sounds like your MC is fine, but the Brake Booster is not working. Bob makes a good point about the vacuum, you need at least 17" of vacuum for the booster to work properly. It is not unusual for a rebuilt Booster to be faulty.

Check the vacuum at your booster. If it does not get to 17", then you either need a vacuum canister, a vacuum pump, or both. If the engine is making enough vacuum, then the booster is not boosting.

 

9/07/2017 1:28 PM  #4


Re: MS GT Brake swap, Master Brake Cylinder defective ?

Bullet Bob wrote:

One thing you don't mention is the manifold vacuum.  You must have 15-20 inches of vac for that booster to work properly. 

I always had sufficient vacuum with the previous Disc/Drum system, never had a problem with the brakes or vacuum, There doesn't seem to be any vacuum leaks and also no auto trans lines or modulator leak problems since I have a manual T5. I changed out the booster for another one just because I thought it could be broken .. no difference after replacement though.

Bullet Bob wrote:

And...I just looked at your pix again and I'm wondering if your "1970 distribution block" is also a prop. valve and causing trouble.  Or, could the slide valve in that thing be off center.

That could be a trouble causer if it were a prop valve but I strongly doubt so, as far as I know, if it came off a drum/drum car, it's just a differential pressure switch . The slide valve gives no signal to be in the off center position either.
Maybe further inspection is needed here, just to be sure.

Bullet Bob wrote:

Also it looks like the flex lines from the MC to the original block make a loop that's higher than the MC piston which could cause bleeding trouble.

I deliberately moved the distribution block 1" lower to make sure the flex lines were not higher than the M/C piston.
The camera point of the photo taken might give it a different appearance but it is on equal height or lower.

Thanks for the input, it's very much appreciated.
Mitch.

     Thread Starter
 

9/07/2017 1:50 PM  #5


Re: MS GT Brake swap, Master Brake Cylinder defective ?

Hornman wrote:

500 psig is about what I would expect with a 1.0" unboosted Master Cylinder. That is enough pressure for drum brakes. Your four wheel disc brakes need about 1200 psig. You said the brake pedal "just stops", which tells us that the MC is not leaking. A leaking MC, either internally or externally, will result in your foot slowly sinking to the floor when you are applying the brakes.

The system is far from 1200 Psi on the rear brakes, I don't think the MC is leaking a lot, it doesn't show either , just a tiny bit of moist near the clip that keeps the piston in. I wonder if there's anything else that could be wrong with it. One of the circuits that fails causing the pedal travel to be longer (?)

Hornman wrote:

It sounds like your MC is fine, but the Brake Booster is not working. Bob makes a good point about the vacuum, you need at least 17" of vacuum for the booster to work properly. It is not unusual for a rebuilt Booster to be faulty.

At first I thought my previous brake booster (9") was at fault so I replaced it with the Reman. it didn't made any difference at all. That's why I do not suspect the booster or believe in a non sufficient vacuum problem as first suspect.

Hornman wrote:

Check the vacuum at your booster. If it does not get to 17", then you either need a vacuum canister, a vacuum pump, or both. If the engine is making enough vacuum, then the booster is not boosting.

The booster seems to be working at least enough to let the pedal drop a little after starting the engine, just like it should, I'll will try to measure the vacuum on the hose to be sure.

Even without sufficient vacuum, the car should still brake fully and the pedal should feel like a normal brake pedal (although a lot more leg pressure is needed) right ?
It's kinda hard to explain but if I have the engine running and I step on the pedal it goes straight 2/3th down with hardly any resistance and then it feels like it hits something on 2/3th , almost as if it was hitting the bottom, or something under the pedal (which it doesn't), rock hard resistance.

Thanks for the input Hornman, it's very much appreciated.
Mitch.

Last edited by Gearz (9/07/2017 2:01 PM)

     Thread Starter
 

9/07/2017 2:33 PM  #6


Re: MS GT Brake swap, Master Brake Cylinder defective ?

You should not have that much pedal travel.  I know the hard stop you speak of. That is the booster or mc bottoming out. You have either slack in the system, like booster output shaft too short, or rear disc brakes not fully adjusted. You MUST have the parking brake hooked up and use it regularly to remove slack in the rear discs. Did you bench bleed the master cylinder? Port closest to firewall going to front brakes?

The power brake pedal on 67-69 cars has 3:1 ratio. Manual is 6:1, so if booster is not working, brake effort will be double.  Just added this since you mentioned it. I doubt the booster is a problem.

If the mc was leaking enough to reduce the pressure, you would see a fluid trail. It should not seep at all. Will replace for you if defective.

Pedal travel should only be about 2" or so at full pedal effort.  A really tight system using those parts can be set up to have 1.5" max travel. Get the pads bedded in before judging the performance, ESPECIALLY if you have plated rotors.  But be sure it is safe to drive before testing. See how to page for tips on bedding brakes.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

9/07/2017 3:21 PM  #7


Re: MS GT Brake swap, Master Brake Cylinder defective ?

MS wrote:

You should not have that much pedal travel. I know the hard stop you speak of. That is the booster or mc bottoming out. You have either slack in the system, like booster output shaft too short, or rear disc brakes not fully adjusted. You MUST have the parking brake hooked up and use it regularly to remove slack in the rear discs. Did you bench bleed the master cylinder? Port closest to firewall going to front brakes?

That is exactly how it feels, something bottoming out in the system.
The output shaft is measured and adjusted with 1/4 turn backwards free play from touching.
Rear disc rotors and calipers are both brand new OEM , pads are new, hardly two miles of road usage since mounted. Master cylinder is bench bleed twice now, ports hooked up firewall side to front.
Parking brake is hooked up, pads are near tight to the rotors, minimum of movement.
How can I adjust the rear brakes / parking brake cables ?

MS wrote:

If the mc was leaking enough to reduce the pressure, you would see a fluid trail. It should not seep at all. Will replace for you if defective.

No fluid trail, no sinking pedal.

MS wrote:

Pedal travel should only be about 2" or so at full pedal effort. A really tight system using those parts can be set up to have 1.5" max travel. Get the pads bedded in before judging the performance, ESPECIALLY if you have plated rotors. But be sure it is safe to drive before testing. See how to page for tips on bedding brakes.

Pedal travel is way more than 2" now, I do know about the bedding brakes procedure but in this case it's not even safe to drive, it should do a hell of a lot better.

Thanks for the quick reply Steve, much appreciated.
Mitch.
 

     Thread Starter
 

9/07/2017 7:38 PM  #8


Re: MS GT Brake swap, Master Brake Cylinder defective ?

To adjust the rears, repeatedly activate the parking brake as hard as you can set it. Three or four pulls should do it.
If still excessive stroke, you have air. I suggest gravity bleeding. It does not aerate the fluid like pedal pump method does.
When bleeding, air can get pulled back through the bleeder screw threads, even if a hose into a fluid container is used. Seal the threads with teflon tape to keep it from doing that. 
There is just no good reaon for a long pedal stroke with four wheel discs unless there is slack somewhere or entrained air.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

9/08/2017 2:31 AM  #9


Re: MS GT Brake swap, Master Brake Cylinder defective ?

I will give it a try and see if it gets any better.
I'm out of the country the next twelve days but will report any results later after returning.
Thanks again.

     Thread Starter
 

10/02/2017 2:06 PM  #10


Re: MS GT Brake swap, Master Brake Cylinder defective ?

So last week, I tried both the parking brake adjustment and the gravity bleed method and after a short little testdrive today immediately noticed the pedal to be better. 
During the bleed procedure I noticed the air slowly getting in along the threads and out thru the hose so I will try again using some teflon tape anytime soon.

Thanks.

https://youtu.be/_VLswF1jF8M
 

     Thread Starter
 

10/03/2017 2:43 PM  #11


Re: MS GT Brake swap, Master Brake Cylinder defective ?

When gravity bleeding, it does not matter if air gets in your hose from the bleeder threads. That is the main advantage of gravity bleeding, other than there is basically very little effort involved.

You could actually remove the bleeder screws and gravity bleed the brakes if you wanted.


Money you enjoy wasting is NOT wasted money... unless your wife finds out.
 

10/13/2017 9:31 AM  #12


Re: MS GT Brake swap, Master Brake Cylinder defective ?

@Gearz
Where did you get the flex lines from your master cyl. to the distribution block?  They look like a great solution for someone who is a newb at bending lines.

 

10/13/2017 10:12 AM  #13


Re: MS GT Brake swap, Master Brake Cylinder defective ?

602Raptor wrote:

@Gearz
Where did you get the flex lines from your master cyl. to the distribution block? They look like a great solution for someone who is a newb at bending lines.

​I had the same problem.  Little experience and tight space.  I spent hours and days bending tubing.  My knuckles were raw from working around the hood hinge. 

Finally,  I called one of the brake companies that advertise in the car magazines - Can't remember which one.  I gave them the specs.  They made me a custom one and got it back to me in a couple of days.  I asked about any problems with flex tubing at the MC.  They said no problem.  Remember the flexible lines going to the front brakes and that one by the rear axle from the body.  Seldom are there problems.

I'd say. Go for it.
 


Original owner - 351w,T-5, 4whl disks, power R&P
 

10/14/2017 7:07 AM  #14


Re: MS GT Brake swap, Master Brake Cylinder defective ?

602Raptor wrote:

@Gearz
Where did you get the flex lines from your master cyl. to the distribution block? They look like a great solution for someone who is a newb at bending lines.

They're Fragola PTFE hoses, ordered them from Summit Racing. 

     Thread Starter
 

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