73 Soft Pedal Learning Process

[Cav73]

I have a 73 Challenger with power brakes and soft pedal. I am reviewing the information about this issue on the forum.

* Restoration of body and mechanical completed 3 years ago; I have been working on the interior. This included removing the complete dash unit and steering column. Those are now back in and the car runs again, so I can finally get to the brakes.

* The car had a 340 originally and came with front disk brakes and rear drums, and a limited slip differential.

* My restorer reused the front calipers, put in all new brake lines, a new power master cylinder and booster from Classic Industries, a rear brake adjustable proportioning valve, and cleaned and rebuilt the rear drums.

* On delivery the car had a soft pedal. The pedal went nearly to the floor before it would stop (which was quite interesting given that the first time stopping it was on a significant downslope into the woods - just barely stopped). It has been in the garage ever since.

* I cannot see any leaks in the braking system but I have not pulled the wheels to check more carefully yet.

* When I started the car the other day, I pressed the brake pedal and it still went nearly to the floor. I think I only put pressure on it once or twice.

* Last night, with the engine off, I depressed the pedal 3 times. The first time it went to the floor, the second time it went half way down, and the third time it was pretty firm.

* Today, with the engine off, the pedal was pretty firm. I could depress it maybe an inch or so.

I just lifted the car and tried rotating the rear wheels in both directions, engine off and e-brake released, clutch in neutral. The wheels do not spin freely, they do rotate with significant resistance. I had to pull and push with some force, but they do move. Researching, more to follow.

 

[pschlosser]

Regarding pedal going to floor:

If there is no brake fluid leak, and the hydraulic system is intact, a pedal going to the floor is typically a worn Master Cylinder (MC).

Air in the system can make the pedal spongy, but going to the floor, even when the car is NOT running, sounds like a worn MC.

If your MC is OEM, and you hope to retain it's aged (and possibly surface-rusted) appearance, you'll need a rebuild kit to set it right, otherwise, a replacement MC may be in order.

All that said, I would bleed the brake system, just to make sure there is no air, and the system is full. I would do this and retest before buying parts.

A worn MC may allow you to "pump" the pedal to get more function. But that's a warning sign the MC needs a rebuild.

 

[Steve340]

With the engine off the pedal should be firm and move to around the same freeplay distance each time you apply the brakes.
Engine on the booster will make the pedal "softer" and move somewhat further.
Sounds a bit like trapped air - and readjust the rear shoes.
All wheels front and rear should move easily.

 

[Cav73]

Pschlosser, Steve340, thanks for the input.

I took some photos today to show the MC, Wilwood adjustable proportioning valve, and the plumbing underneath. I also uploaded the data sheet from the Wilwood site for the adj prop valve that most closely matches mine, as a pdf.

Please note that the MC and booster came together from Classic Industries just a few years ago - apparently the MC was not painted and flash rusted. I plan to paint it with an anti-rust product asap. My restorer assembled the brake system as part of the resto.

I know I have to adjust the rear brakes but I have been looking into whether the adj prop valve might have something to do with the low pedal. I at least want to eliminate the possibility. It is not clear to me what setting the knob on the adj prop valve needs to be at when the brake system is first assembled. However, the data sheet does seem to indicate it should be fully open (full pressure).

Also, the data sheet calls for the hard line for the rear brakes to go from the MC direct to the input port on the adj prop valve. It may not be fully clear in the pics, but the rear brake line actually goes from the bottom of the metering valve, under the MC, up to the adj prop valve. I believe the metering valve in this car is a combo metering/proportioning valve. It looks like the one in the 73 chassis service manual, page 5-17, Fig 3. Hopefully this is not an issue; I plan to call the Wilwood tech line to be sure. My restorer has well over 30 years in auto resto and much racing experience, but I want to make sure I did not ask him to do something that does not make sense by putting in this adj prop valve. I don't see how the rear brake line could come out of the MC separately anyway, but I did not look closely at the MC to see if there is some other port.

I depressed the brake pedal several times today with the engine off. It was pretty firm, but it sits below the clutch pedal about 1.5 to 2 inches - I think I read that they should be about a 1/4 inch apart? I didn't start the car again to test it with power on yet. It is sitting cross-wise in the back of my garage and the fumes become incredible even with both doors open - I really need to swing it around which means moving a lot of stuff!

 

[Cav73]

I found this via google:

https://www.bing.com/ck/a?!&&p=847a...hbHZlK29uK215K21vcGFyJmZvcm09Q1NCUkFORA&ntb=1

Installing a Wilwood Adjustable Proportioning Valve After the Metering Valve on a Mopar​

On a Mopar with a manual 1" master cylinder (e.g., 69 Super Bee, 67 300), the distribution block (also called the metering valve) is part of the factory brake system. It’s mounted on the frame rail and controls the split between front and rear brakes. The residual pressure valve is built into the master cylinder and keeps a small pressure in the rear line for initial brake feel www.forbbodiesonly.com.

Wilwood’s adjustable proportioning valve (e.g., 260‑11179) is designed to replace the factory block and allow you to fine‑tune the front/rear pressure split. It has front inlet/outlets and rear inlet/outlets, and can be plumbed to match your setup Wilwood.

Placement — After the Metering Valve?​

If you’re keeping the factory metering valve (distribution block) and want to add the Wilwood valve, it should be plumbed into the rear brake line after the metering valve. This is because:

• The metering valve already controls the front/rear split and has its own function for the brake warning light.
• Placing the Wilwood valve after the metering valve in the rear line allows it to modulate rear brake pressure without interfering with the front brake circuit www.forbbodiesonly.com.
• If you install it between the master and the metering valve, it could affect the warning light circuit or cause unintended pressure changes www.forbbodiesonly.com.

 

[Challenger RTA]

You need to focus. Pull the drums. I had the same problem just about a year ago. Your are going to find it's a frozen wheel cylinder.
Make sure paring brake is adjusted.

Just had some bake problem on an other car of mine. Did the same thing as yours. Right a way I was going to change the booster and master cylinder. Diagnose first. Is the booster holding vacuum? When at idle can you pull the vacuum hose off the booster and is there vac? Check the check valve see it it is working. Has vac and holding vac. Master cylinder working Does it pump up and can you bleed any fluid out? I could bleed mine but it was only a 50% stream as usual. Is the fluid clean? Even with fresh fluid it was the same. Mine was a little dark. Ended up the line where flushed from the constant bleeding. When moving car very slowly will it stop when using the brakes. Yes mine did but it was not full pedal. Pedal would kind of pump up but when done only about half way up when done.
Ended up finding a rear wheel cylinder was not working. Replaced it and all is good.

 

[pschlosser]

A worn MC will leak fluid around the piston seal, and when (for example) stopped at a stop light, the pedal will slowly go to the floor. If one does not pump the pedal back up, the car can begin to roll forward into the intersection.

The brake hydraulic system is a closed system. If the system is bled, and free of air, then when the brake pedal is depressed, the pressure builds in the system and stops the pedal travel.

The seals on the piston, inside the MC can wear, and when they do, this is the system people find - the pedal going top the floor without providing full brake function.

Having said that, you indicated these brake parts are only a few years old. I am dubious the MC is worn and needs a rebuild. If it's not the MC, I'm not sure what the issue is.

Challenger RTA nails it, though. It may be time to start pulling the wheels and drums, and looking for a smoking gun. Or, take it to a brake and tire shop. This problem doesn't seem unique to Mopars, or vintage muscle cars, so it may be hard for a shop to screw it up.