Testing a brake booster - PeachParts Mercedes-Benz Forum

jalopy_sec · #1 07-05-2009, 11:37 PM

The original brake booster in my 250SE appeared to have a vac leak. (it would hiss when pressing the pedal and I had no power assist), as well as the idle went down when i disconnected the vac line and covered it with my thumb.

I bought another booster on ebay pretty cheap and fitted that last weekend, but it actually seems worse.

My assumption is that the leak would have to come from the vac line or the booster itself, and by testing the vac line with the engine running, it must be the booster.

The master cylender was so rusty and horrid looking that I also put a new one on since the PO had a spare he gave me.

I'm not sure this is a valid test but I tried using a mitivac on the brake booster and i could never get it to hold vacuum. Is this a valid test and is there something else that could be causing this?

The best price I can get for a new booster is $490 so I want to make sure that i've ruled everything else out first.

At least I know how to remove and fit a booster. I think I can do it much quicker now.

Thanks!

Bryce

Gurunutkins · #2 07-06-2009, 12:11 AM

depending on the booster you have an inline vacuum one way valve or a one way vacuum valve in the booster. they go much faster than the booster diaphragm and cause the same symptoms. Basically instead of the brake booster storing vacuum if the valve is shot they never build up a vacuum because on acceleration the one way valve doesn't function and you loose the vacuum. if the one way valve functions properly (you can only suck in one direction) then its the booster
cheers
Barri

jalopy_sec · #3 07-12-2009, 11:00 PM

Thanks.. I tried this and I guess its working.

The hose between the engine and the booster is in 3 sections. There is a short tube between the booster and a small clear pastic resveroir that is capsule shaped. its maybe 5cm long. Then there is a longer section to a black plastic piece (i guess a one way valve) then another section.

If it remove the hose entirely, i can kind of blow through it if i blow through the end on the booster (but its hard and not much comes through), but i cannot suck through it.

Does that sound normal?

Thanks,
Bryce

mbbuff · #4 07-13-2009, 12:54 AM

That sounds about right. I just went through this with my 36 year old 280C (114 Chassis). My symptoms were slightly rough idle causing misfire in front two cylinders (right where the booster vacuum port is located), a slight off-idle flat spot, and a hard brake pedal. It has a vacuum booster pump also. I isolated each component with a custom set of hoses that had a "T" to hook up my vacuum gauge. Areas checked were, manifold to vacuum pump (checks the one way check valve), manifold to booster, bypassing vacuum pump (checks the booster unit), and all hooked up in stock fashion with "T" between vacuum pump and booster (checks the vacuum pump). This way I was able to check each component in the system and record vacuum readings and bleed down times. Using this method I determined that the problem was the vacuum pump. Further checking identified the one way check valve o-ring seals in the pump to be hardened and leaking badly. Replacing them solved the problem. (Cost, $20) My booster is probably the original (I've had the car for 23 years), and the bleed down time is about 30 seconds. If you have hissing with brakes applied, the booster diaphram is probably bad. I also priced a booster unit. My best price was $365.

jalopy_sec · #5 07-13-2009, 01:02 AM

cool. thanks. Time for a new booster I think.

mbbuff · #6 07-13-2009, 10:41 PM

If you're having trouble getting a good deal, or even finding a booster, call Rick at Import Parts Specialists (www.importpartsspec.com). His phone No. 1.800.897.7278. Hope this helps.

jalopy_sec · #7 07-14-2009, 10:48 PM

thanks for the tip!