Brake/booster question

[kerry]

Brakes seem to have gotten harder on the 77 300d. It's holding about 18' of vacuum at idle. Step on brakes, and vacuum drops down about 4 inches per pump and recovers quickly.
How do I tell if the booster is doing a good job?

[barry123400]

I do not know the answer. Comparison of brake pedal with engine on and off might give a small clue. Or if brakes were as hard to use with the booster disconected at it's vacuum source might give another. It does sound like the diaphram is intact and even the valve is working at least partially internally. Unless someone has the diffinitive test like reading the hydralic output pressure from the master cylinder I would read in the archives. Whatever it turrns out to be please post it as even though the master/booster is the same..
I seem to recall more 77 300d owners mentioning this problem. I even have one 77d not in use right now with a harder pedal than I like but have not got to it yet. I want to leave at least one problem not repaired on all my old mercedes. Or why should I start there with so many other possible options? I used to make lists of things I should get to until I ran out of paper. Have reverted to the first come first served at present. Squeeky wheel syndrome? I honestly almost believe as they are hauling me away to the funeral home it will only be in the middle of repairing something. .Or deciding how.

[kerry]

I think the change has been subtle over time. I asked my wife about it and she had been wondering if they had been getting harder to push. I noticed it in comparison with the 85 which I typically drive. My wife drove the 4WD Sidekick today and said the difference in pedal pressure was significant.

I'll try the vacuum disconnect test once the snow conditions improve around here.

[Biodiesel300TD]

To test the booster, put you mity vac on the main vac line, start the car and read the vac gauge. If the vac pump and booster are good the gauge will read 20"-23" of vac. If it's low try pulling the main vac line off the booster, then seal the end of the line where it used to go to the booster, and see if your gauge jumps up to where it's should be. If it does your booster is shot. I think there is an o-ring in it that you can replace. If the gauge still reads low your pump it going.

[kerry]

Originally Posted by Biodiesel300TD

To test the booster, put you mity vac on the main vac line, start the car and read the vac gauge. If the vac pump and booster are good the gauge will read 20"-23" of vac. If it's low try pulling the main vac line off the booster, then seal the end of the line where it used to go to the booster, and see if your gauge jumps up to where it's should be. If it does your booster is shot. I think there is an o-ring in it that you can replace. If the gauge still reads low your pump it going.

It reads steady at 18" or so. I think that is the result our altitude--a mile high.

[toomany MBZ]

It will take many adapters, but you can hook up your Mity Vac to the booster, and pump, and pump, and pump, and pump some more, it will eventually come up, if it holds it's good, otherwise, there is an "o" ring that can be replaced easily, (remove master cylinder from booster, you'll see it) check again. These boosters run near $300. Good luck.

[kerry]

There's clearly something wrong. It's taking double or triple the pedal effort to stop the 77 compared to the 85. What could be causing this lack of force from the booster if the vacuum seems ok? It's holding vacuum in the system, the booster seems to consume 3 or 4 inches with each pump, but then the system recovers. At the pedal it doesn't seem that the booster is doing its job but under the hood the vacuum seems to be working.

[kerry]

TTT

Any other hypotheses?

[dougk]

Originally Posted by toomany MBZ

It will take many adapters, but you can hook up your Mity Vac to the booster, and pump, and pump, and pump, and pump some more, it will eventually come up, if it holds it's good, otherwise, there is an "o" ring that can be replaced easily, (remove master cylinder from booster, you'll see it) check again. These boosters run near $300. Good luck.

I did the pumping, it does take a loooonngg time. But I found that my brake booster held it's vaccum. That is the reason I found my vaccum pump was bad.

On a side note:
Can someone explain how come the brake booster looses vaccum when it is activated? Example: the vaccum pressure drops when the brakes are used. Where does the vaccum go?

DK

[TerryS]

If you're holding vacuum, your problem may be the master cylinder, not the booster. You could have a plugged orifice (no wise cracks here) Disconnect each circuit, at the MC, one at a time a depress pedal.

[kerry]

Replaced the front brake pads and solved the problem. They were quite worn. Perhaps an 1/8" left on the thinnest one.

[barry123400]

I remembered when you mentioned this that the caliper will not extend enough to activate the pads. Or limit the action when the pads are way down. Could be a design thing. Anyways it was talked about before. Hindsite is so beautiful. Also this is something that probably should not be.

[kerry]

The odd thing is that I jacked up the car to see if the front brakes were working and they stopped promptly when I stepped on the brakes. No visual distinction between the front and back brakes. This is what had made me put off replacing the pads for so long. But something different was definitely going on.

[Biodiesel300TD]

Were you spinning the front wheels by hand? If so there isn't much force behind that when you do. You arent' actually stopping the car, you are just stopping the wheel. It doesn't take much to stop the wheels, whereas it take a lot more to stop a 2500 lbs car traveling down the road.

[toomany MBZ]

Originally Posted by kerry edwards

Replaced the front brake pads and solved the problem. They were quite worn. Perhaps an 1/8" left on the thinnest one.

Glad to hear you got your problem solved. Yet the service limit on the pads are 2mm, about a 1/16 right? But the pads may have had issues.