Help - Brakes just got really hard to press! - PeachParts Mercedes-Benz Forum

Gil · #1 10-24-2007, 02:33 PM

I think I lost vacuum in the brake booster or a line leading up to it. I was driving down the highway yesterday and went to push on the brakes. The pedal got extremely difficult to apply, though I was able to get the car stopped. There were no hills anywhere and I was not using the brakes much before that, so I ruled out overheating as a culprit. I immediately stopped by my folks house and went to shut the car down, but had to use the stop switch under the hood to do this. It's sitting there till I figure out what to do.

I think the vacuum valve behind the fuel pump is shot, since the car had a habit of taking an extra half second to shut down once turning off the key. Now that the key will not shut off the car and I must use the emergency stop switch, my guess is that this and the brake booster vacuum problem are related.

Am I right in assuming this and if so what part would I need? The car is an 84 300CD.

Thanks

Dex · #2 10-24-2007, 02:46 PM

I'm no expert auto mechanic, but I had the same thing happen to me 6 months ago. In my case the vacuum pump itself had failed. The results were the same. I could brake if I press the pedal hard and I had to stop the car with the emergency stop switch.

Oh, and I did end up replacing some of the rubber joints on my vacuum hoses because some of them looked very suspect.

dieseldan44 · #3 10-24-2007, 02:49 PM

You have a massive vacuum leak OR your vacuum pump is not making any vacuum. You can disconnect all of the accessory lines from the main brake vacuum line and the booster will still work fine under normal conditions.

I'd check the vacuum pump first and make sure its working - it may be spewing oil into your vacuum system if its broken!!!! dieselgiant's website has a great pictorial on vacuum troubleshooting that shows how to test the vacuum pump.

You can test the booster for leaks - I had a thread a while back where dmorrison walked me through it (and replacing my booster after it failed the test).

dd

Gil · #4 10-24-2007, 02:49 PM

The vacuum pump is that thing on the front of the engine, right? Is it a @#$!! to replace (assuming that's what's wrong)?

Phil · #5 10-24-2007, 02:50 PM

I'd try disconnecting the vac line to the stop valve and plug the line then see if the brakes work, if so the I'd guess the stop valve, if not then start disconnecting and plugging other stuff until the brakes work. Only disconnect one thing at a time so you know what is causing the problem.

Gil · #6 10-24-2007, 03:56 PM

Thanks, I have a feeling it'll be the stop valve; will find out. How much of a PITA is it to change the stop valve?

ImBroke · #7 10-24-2007, 09:07 PM

Just changed my vac pump, wasnt that hard. I made it easier by removing my AC compressor. Only difficult thing was I stripped out an allen head bolt, and had to use a vice grips to get it.

Dex · #8 11-26-2007, 02:16 PM

I replaced my vacuum pump a few months ago, it wasn't that hard to do. Biggest pain was scrapping the old gasket off. There was some question as to whether or not to glue the gasket, though. The answer is DON"T glue it. It's apparently an almost garaunteed vacuum leak. I didn't glue it on and it worked like a champ.

Gil · #9 11-26-2007, 02:52 PM

Turned out it was a vacuum pump, thanks!