Weird Problems with Power Brake Conversion. Need Help.

[68Coupe289]

To recap, I'm going from 4 wheel manual drums to power front disks on my 68, with all mustang parts. I obtained disk brake spindles and distribution block from a donor mustang, and bought everything else new. New booster, Master cylinder, Adjustable Proportioning valve, Power Brake pedal, calipers, lines, etc.

We bench bled the MC, and bled the system thoroughly.

Here's what I am experiencing:

1. The car will stop fine, but the pedal feels very soft with a lot of travel. I've never driven an old Stang with power brakes, so I have nothing to compare, but it doesn't feel right.

The pedal seems to have great pressure and feel, until I start the car. Then it goes soft.

2. Bigger problem: The pedal will not return on its own, and the brakes stay engaged until I pull the pedal up with my foot. WTF?


As I said, we bled the system well, over and over again. There are no leaks. I have a vacuum line running from the booster to the intake. I put the adjustable prop. valve where the stock one would have gone, in line between the 2 rear ports on the distribution block (the power dist block has 2 extra recepticles in the back). No amount of adjustment on it changes the problem. I checked my restoration guide, and made sure the valve is orientated the right way according to IN and OUT (bottom of the dist. block to IN, and OUT to the top of the dist block).

I used DOT 3 brake fluid.

Did we miss something? Do you have to lengthen the MC pushrod, even if you are using all the original spec parts, including the brake pedal? I left the back drums alone (still stock). Does anything need to change back there?

We are lost as to what to do to fix this. Thank you all so much for any advice. I'll be searching for answers in the meantime.

 

[WORTH]

To recap, I'm going from 4 wheel manual drums to power front disks on my 68, with all mustang parts. I obtained disk brake spindles and distribution block from a donor mustang, and bought everything else new. New booster, Master cylinder, Adjustable Proportioning valve, Power Brake pedal, calipers, lines, etc.

We bench bled the MC, and bled the system thoroughly.

Here's what I am experiencing:

1. The car will stop fine, but the pedal feels very soft with a lot of travel. I've never driven an old Stang with power brakes, so I have nothing to compare, but it doesn't feel right.

The pedal seems to have great pressure and feel, until I start the car. Then it goes soft.

2. Bigger problem: The pedal will not return on its own, and the brakes stay engaged until I pull the pedal up with my foot. WTF?


As I said, we bled the system well, over and over again. There are no leaks. I have a vacuum line running from the booster to the intake. I put the adjustable prop. valve where the stock one would have gone, in line between the 2 rear ports on the distribution block (the power dist block has 2 extra recepticles in the back). No amount of adjustment on it changes the problem. I checked my restoration guide, and made sure the valve is orientated the right way according to IN and OUT (bottom of the dist. block to IN, and OUT to the top of the dist block).

I used DOT 3 brake fluid.

Did we miss something? Do you have to lengthen the MC pushrod, even if you are using all the original spec parts, including the brake pedal? I left the back drums alone (still stock). Does anything need to change back there?

We are lost as to what to do to fix this. Thank you all so much for any advice. I'll be searching for answers in the meantime.

ONe thing I noticed from your post, you said you put the proportioning valve between the 2 rear lines, you should only have one rear line and 2 front lines/

 

[68Coupe289]

On the power brake distribution block, the left front brake comes out the front of it, and the right front brake comes out the front bottom port. The rear brake line comes out of the middle bottom port, and the MC goes into the 2 front top ports.

There are 2 more ports on the power distribution block in the back (top and bottom). My restoration guide shows that a line goes from the bottom port, through the stock proportioning valve, and into the top rear port (so it forms a closed loop). Instead of using the old stock proportioning valve there, I used a new adjustable unit, which is supposed to be a direct replacement.

 

[WORTH]

On the power brake distribution block, the left front brake comes out the front of it, and the right front brake comes out the front bottom port. The rear brake line comes out of the middle bottom port, and the MC goes into the 2 front top ports.

There are 2 more ports on the power distribution block in the back (top and bottom). My restoration guide shows that a line goes from the bottom port, through the stock proportioning valve, and into the top rear port (so it forms a closed loop). Instead of using the old stock proportioning valve there, I used a new adjustable unit, which is supposed to be a direct replacement.

Sorry, I misunderstood. Well it is normal for the pedal to feel a little looser when the engine is running, however it should return fully. Did you adjust the rears up good? They may be traveling too far.

 

[ForceFed70]

Pedal doesn't come back up? Weird... I think there's a spring in the master cyln that causes the backpressure and the brakes to return. I wonder if the proportioning valve is causing issues with letting the fluid flow back into the master cylinder.

Why did you install a proportioning valve if your using a stock setup? Most people only do this if they are using a system made for 1 type of brake and switching to another (IE: master cyln for 4 wheel drums, but swapped to disk in the front).

 

[68Coupe289]

Pedal doesn't come back up? Weird... I think there's a spring in the master cyln that causes the backpressure and the brakes to return. I wonder if the proportioning valve is causing issues with letting the fluid flow back into the master cylinder.

Why did you install a proportioning valve if your using a stock setup? Most people only do this if they are using a system made for 1 type of brake and switching to another (IE: master cyln for 4 wheel drums, but swapped to disk in the front).

The 68 mustang power brake stock factory setup has a proportioning valve in the system. You have to have one if you run drums in the back.

I only put an adjustable in because, to my knowledge, the original style valve is not available new. I have the old one from the donor car, but it looked pretty rough, so I didn't use it.

I checked my drums. The driver's side rear is too tight, and the other is way too loose. I'm going to break those open this weekend and have a look.