'72 Polara Brake Story, Pics., Questions

[72DL41]

Story:
- '72 Polara base model survivor (20k mi.); factory power disc/drum (Broadcast Sheet Line 7 codes: front size B07, rear size B27)
- Master cylinder tag numbers/letters: 1176; 3812A; PWR
- When I purchased the car five months ago, I checked fluid levels, lights, tire pressures, etc. The master cyl. level was good.
- Since then, I've put about 200 mi. on it. The brakes always felt "touchy" to me (not much pedal pressure required). I assumed that was normal.
- On my way to a cruise night last week, I had to make a "panic stop" and the rear brakes locked up and the rear end started to come around.
- Last Friday, I was checking the car again before a trip to the Gilmore Museum on Saturday. The m/c front reservoir (rear brakes) was almost empty; the rear reservoir was good.
- The brake warning light never came on when I did the "panic stop."
- I looked the car over for any signs of leaks (m/c, combo. warning/prop./metering valve, lines, hoses, back of the tires, etc.); no obvious leaks.
- When I apply the parking brake, the brake warning light comes on and goes out when I release it.
- The '72 FSM says to check the brake warning switch operation by opening a bleeder and step on the brake pedal. I assume I can skip that because the light works and I need a new switch.
- As far as I know, there's never been any brake system work done. It's still got the "nipples" on the bleeder screws. I'll probably have the rear drum assemblies inspected.
Questions:
- Any thoughts on where the fluid may have gone? I've heard of fluid "bleeding" into the booster, but wouldn't that affect the rear reservoir also?
- Based on the B/S codes, numbers, letters, and pics., what brake system do I have? Bendix large single diaphragm, 11x2 3/4" disc, floating caliper, 11x2" drums, 1-1/32" bore m/c?
- Combo. warning/prop./metering valve assembly number 3583635. Confirm? Source?
- Can the brake warning switch be replaced? Part number? Source?
- Is there a source for pre-bent lines and hoses if I ultimately need them?

Thanks for your input.

 

[Big_John]

The brake warning light never came on when I did the "panic stop."

It's not supposed to. It is only supposed to light up if you have a hydraulic failure with the brake system. Or.... See the next.

When I apply the parking brake, the brake warning light comes on and goes out when I release it.

That is working like it supposed to. The brake light doubles as the E-brake light, telling you the E-brake is on.

The '72 FSM says to check the brake warning switch operation by opening a bleeder and step on the brake pedal. I assume I can skip that because the light works and I need a new switch.

Why do you think you need a new switch? That seems to be working fine.

As far as I know, there's never been any brake system work done. It's still got the "nipples" on the bleeder screws. I'll probably have the rear drum assemblies inspected.

I don't believe that they ever put the rubber "nipples" on the bleeders when the car was built, but I could be wrong.

Any thoughts on where the fluid may have gone? I've heard of fluid "bleeding" into the booster, but wouldn't that affect the rear reservoir also?

The only way to really check for leakage of the rear wheel cylinders is to pull the wheels and drums. If the brake shoes are soaked with fluid, it can cause them to grab and lock, which could cause the problem you had with your panic stop.

But, if you are used to driving newer cars with anti-lock brakes, the rears locking up first may be something you've never experienced. What happens is the weight goes forward and makes the rear lighter, making it just a little easier to lock the rears. A little history of ABS is that GM and Ford's first ABS systems were rear brakes only, not all four wheels. GM was 1969 and Ford was 1971. In 1971, the Imperial was the first production four wheel ABS.

 

[72DL41]

Thanks for your response.

- I assumed one of the m/c reservoirs being empty would be considered a hydraulic failure and turn the warning light on. That's why I thought I needed a switch.
- I thought the "heavy" brake pedal pressure during the panic stop could have possibly caused the wheel cylinders to start leaking.
- If the rear wheel cylinders were leaking enough to empty the reservoir, I would've expected to see fluid on the back of the brakes or tires. I'll check them.
- Using the FSM switch testing procedure, how much fluid needs to be bled for the light to come on?
- All of my previous classic cars were drum brakes, except for my '62 Savoy which was manual disc/drum and my '90 Dakota, which had rear ABS.

 

[Big_John]

I assumed one of the m/c reservoirs being empty would be considered a hydraulic failure and turn the warning light on. That's why I thought I needed a switch.

While the reservoir may look empty, you must still have enough fluid in the system to work the rear brakes. The switch only comes into play when there is no pressure on one of the circuits. It doesn't measure how much fluid is in the system. It works using a spring loaded valve that if there is a large differential in pressure between the two brake circuits.

I thought the "heavy" brake pedal pressure during the panic stop could have possibly caused the wheel cylinders to start leaking.

Usually a weak hose or line will break (pun not intended LOL) under heavy braking, not a wheel cylinder.

If the rear wheel cylinders were leaking enough to empty the reservoir, I would've expected to see fluid on the back of the brakes or tires. I'll check them.

Not necessarily. Yes, you might see that with a bad leak or failure, but you probably won't see it with a slow leak like you have.

Using the FSM switch testing procedure, how much fluid needs to be bled for the light to come on?

I wouldn't bother. You'll know from the pedal before the light comes on. You haven't done anything to have the light turn on.... In other words, you still have pressure in both circuits. Lose pressure one side and the pedal will get very soft.

Testing it means opening up the bleeder so there's no pressure on that half of the circuit. Any fluid you would lose is whatever squirts out of the bleeder. Keep in mind, this means you will have to bleed the brakes because you've probably sucked air back into the system when the pedal is released. Actually, you'll probably see it work when you replace the rear wheel cylinder (if that is the problem) and have to bleed the brakes.

I think you have to understand that the switch doesn't care how much fluid is in the system, it just looks at the pressure differential between the two circuits. You haven't lost enough fluid (yet) to lose pressure.

 

[72DL41]

I appreciate your input and thanks for helping me understand the system. The information may help other members also. I'll have the rear brake components checked.

 

[CBODY67]

Some modern master cyls have a fluid level sensor to alert of low fluid, not specifically "empty", just low. Where is the sensor in your system for that?

If a rear wheel cyl is leaking, it will leak onto the brake linings first, which can cause some grabbing in any stop, as I recall. Such a large leak can also result in any brake fluid leaking also getting on the inside sidewall of the tire, with fluid trails resulting from centrifugal motion of the brake drums. A small amount is normal, as I recall, like a drop or similar, but anything more can mean the cyl needs to be rebuilt (which is easier, but messier, than replacement with "modern" units).

With such low miles that you've owned the car, you are still in the "learning phase" of getting a good feel for the car, how it acts, what it wants, and how best to extract its best performance. So far, what've you've been concerned about is more normal fo the cars than not.

As to the brake performance, you'll feel the pedal take more travel to stop the car when a fluid leak happens. Before any light illumination of a complete failure of one end of the brake system. Consider the instrument panel "BRAKE" light to mean, when illuminated, as when to NOT drive the car. The parking brake could be partially applied or one end of the foot brake system could need attention (due to loss of fluid).

DO watch the brake fluid level, every so often. Also inspect each of the brake flex lines (one at each front wheel and the one at the rear axle) for fluid seeps, too. For general principles, you might start looking for some NEW, NOT NOS, brake lines. "NOT NOS"? Yes, rubber ages and dries out, so the newer it is can be better insurance against future failures. Quality OEM-level brands, too.

Might not hurt, too, to do a full brake system flush to get the OEM fluid out and new, "dry" fluid in. "Dry" meaning fluid that has not had a change to absorb moisture from the atmosphere.

In the mean time, get to better know the car. How it responds to steering input, brake pedal input, and throttle input. Learn what works best in what situations. Then use that information to learn to drive it so it and you enjoy driving.

Don't forget to change the rubber fuel lines, from the tank FORWARD to the carburetor. Ethanol'd fuels will cause them to seep, leak, and then fail. If they might start to seep, it can be hard to track a fuel smell as the evaporated fuel usually leaves little, if any, trail.

Enjoy!
CBODY67

 

[72DL41]

CBODY67:
- The switch is part of the combination warning/prop./metering valve assembly below the master cylinder.
- I looked the car over for any signs of leaks (m/c, warning/prop./metering valve, lines, hoses, back of the tires, etc.
- I might do the hoses and system flush when I get the rear brakes inspected.
- The rubber fuel lines/hoses are on the "to do" list.
- Yes, I'm getting familiar with the "feel" and operation of my "new to me" C-Body.

Thanks for your input.

 

[CBODY67]

Thanks for the additional information.

I'm aware of the switch and its location. It is only a pressure-bias switch and no more. ONLY for when one circuit loses complete pressure, not partial pressure. Seems like there is a procedure to "re-center" the switch after a brake bleeding operation?

Of course, when that circuit loses pressure, the pedal travel will be much greater to stop the car! Kind of redundant, but some people need to see the light to see that something is wrong.

CBODY67