I have most of the bugs worked out on the 1991 350SDL since coming out of hibernation. Thanks to everyone here.

I have been taking it on shake down runs around home to see what pops up with out any real issues. These have all been short two or three mile trips at about 25 to 35 mph. Then back home to see if I have any leaks or other abnormalities.

Until two days ago. That was the first trip off island down to my wife's work to pick her up. It is around nine miles each way doing around 45-55 mph. About half way there I lost power brakes. After I picked her up we drove about two miles and decided to stop for happy hour. The car sat in the parking lot for about an hour and a half. We started to head back and still no power brakes until about 5 miles from home they mysteriously came back. I thought whatever, it's been sitting awhile. Maybe they just decided to take a break for a little bit.

Today, same issue. I made a couple trips around home. Mostly to NAPA with out incident. Then I had to drive a few islands over, about seven miles each way. About two miles in I lost all power brakes.

It still stops ok but the pedal goes down further than it does when the power brakes are working. But it does scare you when they disappear without any notice.

What would make them go away them come back on there own? I don't know if it's heat related or that is just a coincidence. Where do I start looking?

Thanks in advance
Robert

Iam no expert, but sounds to me like you have a vacuum problem.

Check the vacuum with your Mity-Vac on the big black line from the vac Pump to the brake booster.
Should be around 22HG.

If there is a way to "T" in a vacuum line from the big black line, into the cabin with a vac gauge, you could see if the vacuum falls when you have the problem.

I had this same problem with my 86 F250 6.9 Diesel. Peddle got hard as a brick, and went low. Checking things out, I had no vacuum from the pump.
Replaced it and all was good.

It was a bad time for this to happen, but luckly I was pulling into a parking space for a Burger.
We were hauling the 300SD on a trailer, had the car packed, 5 617.652 engines, 6 MB manual gear boxes and 4 or 5 MB diffs in the truck bed.
We were moving from Ca. to Nv.

Charlie

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We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Calipers do not sit unused well in many climates and this car has been sitting for a long period. So first things first.

Drive the cart a mile or two. Get out and check each individual wheels temperature. Wet your finger or use a temperature reader. Sometimes you will even smell the heating.

Also change out the old brake fluid. You may just be boiling the old brake fluid that may also contain a lot of moisture.

Although the instance of the car sitting for awhile and still having no brakes when you came out of the restaurant. I have no knowledge if this could also be related. May depend a lot on how long you where in the restaurant. It is still possible though.

Or jack each wheel off the ground and check manually for excessive caliper drag. As I mentioned usually with a car sitting a long period of time caliper issues are kind of to be expected. You should always more or less do this check on any brand of car with disk brakes.

Why I preffer the drive test. An old brake hose that is only partially collapsed internally might be involved. This is perhaps a better way to catch it. Even if this is not the problem and it may very well not be. Disk brake calipers just do not in general do well again when just sitting around for really long extended periods. Especially with perhaps twenty year or more old brake fluid in them.

With such incidents I would think about installing vacuum gauge.
One of indicators of low vacuum in the system is cabin air going into defrost mode, what is default.
The vacuum pumps of the era had diaphram, what after 25 years might need replacement.

If you lose vacuum, the engine won't shut-off (use emergency "stop" lever) and the door locks won't actuate. As mentioned, T a vacuum gage into the cabin to monitor as you drive. Need at least 15" Hg. Could simply be a leaking rubber hose.

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1964 & 65 Mopar's - Valiant, Dart, Newport
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For starters, I find it incomprehensible that you are still driving a car..ANY car with failing brakes. Think kid jumping out in the street chasing a ball and you cant stop !

As suggested.. check your vacuum, change your brake fluid. IF that doesnt fix it, have it towed to a shop that can fix it.

Know failing brakes...and you are still driving it??? What the hell are you thinking??

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First off, I am not still driving the car!! So relax a little bit. It is a second car and it has been sitting since that trip. The brakes still stopped fine I just didn't have power assist. Maybe ask before you start flaming ppl please.

Before I drove it I checked each one of the calipers. I took them off and checked that both pistons moved out freely and that I could push them back in without hanging up. I had one that was stuck and after it was exercised once it moved freely on each successive try.

It is waiting for new pads that i ordered for it front and rear with all new associated hardware to show up from the Fedex man today. The pads are still well within the acceptable range but getting a little low. The hardware is a little rusty so since I'm taking it apart it's getting new pads. I will be replacing the brake fluid at the same time. It doesn't look bad but it has been sitting so better safe than sorry. The rotors don't have many miles on them and still look like new. I just wanted to do both jobs at the same time.

I have been changing the transmission fluid and filter and she got all new fuses while I have been waiting.

I have a couple vac gauges. Does anyone know where the van line comes into the cabin so i can tap into it?

Thank you again everyone for the help.

Hard pedal -> check vacuum, check booster, check vaccum accumulator.
Power lock and shutdown issues are symptoms of vacuum probs as well.
Pedal sinks-> Look for leaks.
Pedal pushes back so hard then sinks (breaks, like a trigger) -> Replace master cylinder.

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You're four times

It's hard to

more likely to

concentrate on

have an accident

two things

when you're on

at the same time.

a cell phone.

www.kiva.org It's not like there's anything wrong with feeling good, is there?

Don't know your model, but all the designs have one principle.
Vacuum pump has big pipe that goes toward brake booster and somewhere on that pipe is a Tee where small pipe/tubing goes to car accessories.
In case of vacuum leak on accessories the small tubing will not allow major leak and you will still have boost.
In your case you lost boost, so it is either leak on big pipe, bad pump, or bad booster.
I would tap gauge right at the main Tee so you can monitor vacuum at boost pipe.

Sounds like you cleared the potential sticking caliper issue earlier. Brake fluid looks good is great. It would have taken a sticking caliper to have boiled it usually anyways if it had only a little moisture.

Why I suspected something like the above was the car had to be driven a distance before the issue occurs. The only other thing I can think of distance related is perhaps the vacuum pump dropping out. Although this is not certain either.

Most brake problems I have experienced are not distance travelled related. They have just been constantly bad until repaired.



Our daughters 2014 ford fusion was always back in the dealership. They could not seem to fix the issue and usually had the car for days. She finally got fed up and traded the car.

When she turned up visiting us with her new replacement Toyota. She told me the last time the fusion was in for service they told her she needed new pads and rotors. The car had six thousand basically almost all highway miles.

It was too late unfortunately at that point but I suspected that car had perhaps intermittent dragging brakes. This may have been the reason the garage told her the cars issue was boiling the motor oil. Also she had mentioned the cars fuel milage seemed to be pretty poor to what she thought it should be.

I will never know for sure what the problem with that car was. I do think the worn out brake issue was really abnormal though and may easily have played into the situation. Another clue was the engine always overheated on a very hilly isolated stretch of highway on the way to work. The dealerships mechanics just may have missed it was intermittent brake drag related.

That daughter never drove with one foot on the gas pedal and one on the brake either. I had taught her to drive 20 years earlier. Plus ridden in cars with her driving many times over the years.

I'm with you Barry. I have never had a brake issue that was distance related either. In my experience they are either bad or not. That is why I was thinking the vac pump may have issues.

Has anyone had a vac pump work intermittently before? Do they have a tendency to fail after they get warmed up then start working again after they cool down?

Btw: The car did shut off with the ignition key when it was acting up. Can it have enough vac to shut off but not enough for the brake booster to work properly?

Thanks Kajtek1. Thats good info. I will tap right at the vac pump.

One thing that I missed is I believe you car has abs brakes. If and how this may or may not play into the picture is beyond my knowledge. I suspect it has no bearing at all yet at the same time just do not know enough.

What you do have to check if and when you change your brake fluid is the best way to do it. Some abs cars are more complex and some not so much to change the brake fluid on I think. My guess is you already know this.

I also am too unsure of what your exact issue is to make good suggestions. My 77 300d has lost all power assist on the brakes as well. I just have not had the time to look into it. The car is also not in service. I did pick up a used booster for almost nothing for when I do get into it. I believe my car has known good vacuum though. There is a check valve in the vacuum line to the booster. This is so the booster can retain enough vacuum for more than one stop with boost. If say the engine quits I suspect. I am not sure what failure of that valve would cause. I am not suggesting this is your issue either. It was more just a question that arose in my mind..

I vote for seized calipers.

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I just checked my vac. My car does have a tee inline with the large line going to the booster so I tapped in there as suggested. I have 24" as soon as I hooked the gauge up and 27" after about two minutes. I have the gauge plumbed where I can see it from inside the car so that I can monitor it while driving. But I won't be able to take it for a ride for a couple of hours yet.

Yes, my car does have ABS. I don't see how there could be a connection either. But it's worth a thought. I will look to see if there is a special procedure for changing the brake fluid.