Wonky brakes on my 300td..

[a1anda2anda3]

I've been driving my 84 300td wagon a lot recently--4000 miles this summer, or so. Occassionally, when I step on the brake, it goes almost all the way down to the floor, as though I nearly lost all brake function, but if i pump it once it comes back. It's occasional. The normal condition these days has been that the brakes are hard and it requires some leg strength to stop the car, but the stopping ability is there. Maybe I have multiple things going on. What I know:

-Vaccuum system is wonky, but i've teed off a number of things that could be leaking such as locks. Do hard brakes mean the vacuum booster isn't working?

-At no point have I seen any leaks of brake fluid from anywhere. The fluid should be replaced & bled, its been a few years. I assume because the fluid level has remained normal that the master cylinder wouldn't be leaking into the booster?

-Calipers don't appear to be dragging and seem to grab and release the rotors appropriately.

Where should I start? Want to keep driving the wagon, but of course want to avoid crashing it should this behavior get worse!

[Diseasel300]

Brakes are one of those things that should NEVER be ignored, but often are. It's a safety issue for you and anyone else you encounter on the road!

Hard brakes mean the booster is not receiving vacuum, or if it is, the diaphragm is torn and not holding. You already stated you have vacuum problems, get those fixed and see if that problem resolves itself.

Brakes going to the floor usually indicate a leak, air ingress, or a failing master cylinder. Do the basics first, look for wetness, bleed the brakes (and replace the fluid), and see if the problem resolves. If it does, great, if not, time for a new MC.

[a1anda2anda3]

Thanks for your reply!
I haven't been able to find any brake fluid wetness.. i looked at each wheel and its brake hose, and the reservoir level hasn't changed basically since I bought the car. I'm certainly going to take your advice and replace the fluid and make sure it's bled. I was guessing that occasional extreme softness of the pedal that I can pump through seemed, at least intuitively, like air bubbles. Can air get in to the brake system without fluid leaking out? (or is it anyones guess..?)

[Diseasel300]

Brake fluid is hygroscopic (absorbs moisture from the air). If the car has been neglected and the fluid not replaced every couple of years, it can absorb enough moisture to boil the water in the fluid when the pedal is pushed. That will cause a soft pedal. Loose fittings or pinholes in the brake lines will also allow air into the system. If it hasn't been done, flushing the lines and replacing the fluid is the first step, especially if it's been years.

[firstdiesel]

Hi guys, I gotta say I've had this exact problem and I posted about it a little over a year ago:

84 300TD Brake question

Sadly I did not have the resources at hand to fix the problem as my house is just finishing a construction job and I have not had access to my tools. Yes, it has taken over a year unfortunately.

a1anda2anda3 I am in this with you and I would like to get to the bottom of this as well. I can say that I did replace the caliper seals around the entire car and I was thinking that one of the valves could be stuck (rear left to be specific) possibly causing the hard brake pedal. I do not have vac issues, all door locks work, ac vent pods work and car shuts off promptly. When the pedal goes almost to the floor for a brief time the brakes work as I would expect them to with easy touch braking so I am not sure it is the booster. MC maybe and definitely it could be the fluid.

I too will change the fluid and the rear rotors and see if that works as well. It has been too long since the last fluid change.

This is a new one for me so I appreciate the help Diseasel300. Stay tuned!

Todd

[Texasgeezer]

You might think I'm whacko, as a few others do, but I recently found the ABS unit can cause brakes to fail even with normal braking on dry surfaces. A few BMW and Porsche enthusiasts discovered that their ABS units were contaminated and interfering with braking action. Sometimes one wheel, sometimes two, and possibly all four. The ABS solenoid valves can block brake pressure if they are not clean and functioning properly

This page has links that include narrative and photos that show the innards of ABS units and tiny filters that can trap particles.

1995 E300D, Brake fluid flush with pressure attachment at master cylinder cap

One of the links will take you to a site that shows an unexpectably high ABS failure rate due to age and mileage.

The ABS is a typical non-service component. Owners have no clue that it might cause unexpected brake problems.

If I had an old car with ABS I would either send the unit to BB-reman for checkout, clean the ABS myself and test it, or remove it completely and drive with non-ABS brakes (like the old days). At least without the ABS unit the brakes would work properly as long as the other components are in good condition.

[Diseasel300]

W123's don't have ABS.

[Texasgeezer]

Optional in August of 1980

[Diseasel300]

Originally Posted by Texasgeezer

Optional in August of 1980

A very EXPENSIVE option that precious few cars came with.

Worrying about the ABS pump is like worrying that the sky is going to fall. If it fails (extremely rare), so what? You still have normal brakes. It isn't like the later SBC system that you're screwed when the pump dies.

[Diesel911]

Originally Posted by firstdiesel

Hi guys, I gotta say I've had this exact problem and I posted about it a little over a year ago:

84 300TD Brake question

Sadly I did not have the resources at hand to fix the problem as my house is just finishing a construction job and I have not had access to my tools. Yes, it has taken over a year unfortunately.

a1anda2anda3 I am in this with you and I would like to get to the bottom of this as well. I can say that I did replace the caliper seals around the entire car and I was thinking that one of the valves could be stuck (rear left to be specific) possibly causing the hard brake pedal. I do not have vac issues, all door locks work, ac vent pods work and car shuts off promptly. When the pedal goes almost to the floor for a brief time the brakes work as I would expect them to with easy touch braking so I am not sure it is the booster. MC maybe and definitely it could be the fluid.

I too will change the fluid and the rear rotors and see if that works as well. It has been too long since the last fluid change.

This is a new one for me so I appreciate the help Diseasel300. Stay tuned!

Todd

If you have ABS and there is valves I have no experience with that. On the non-abs brake system there is no valves once you are outside of the master cylinder.
People have said that the inside of Brake Hose deterioration can act like a valve.
That water that is absorbed into the Brake Fluid goes to the bottom of the Master Cylinder and the Calipers and creates pitting and a sort of a muddy muck.
That muck at the bottom of the master cylinder can help to fill in the pitting and make for a better seal then the pits do but that can be intermittent.

That muck in the Calipers eventually contributes to the Caliper Piston not returning normally.
That much can cause pistons to stick (also inside the Master Cylinder) and block ports even of only temp.
The Piston on the front portion of the Master Cylinder has a valve in it and it returns only by spring pressure.

It is possible for the Master Cylinder to leak fluid out of the rear end where it goes into the Power Brake Booster with no leaking.
If you did see leaking from that area it would mean that the O-ring between the Booster and the Master Cylinder is no good and that should cause a vacuum leak.

[Diesel911]

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