problem with brake booster

[lurtch]

Hello to all - - If you please,  who  among us is knowledgable about the brake vacuume servo and the non-return valves?

All of a sudden I need an enormous amount of pedal pressure to get this thing stopped. No leaks are apparent, no squeal, no pedal vibrations - - all four corners have new or within tolerance calipers and  pads- - .  The Haynes manual is not difinititive about the non return valves.

Are these things simply a check valve? Do they need cleaning periodically?  My car may be unique and incorrect because of  inept tampering episodes by P.O.s.  There are TWO non-return valves in the line over from the manifold. Could this be a contributing factor? Do they clog in the open position? Is there another priority valve inside the diaphram chamber? Wouldn't a ruptured diaphram emit a noticable hiss?

Thank in advance - - - Larry

[rwmastel]

Larry,
There should only be one check valve in the line from the intake manifold to the booster.  You can simply take these out and test them, possibly with your breath??  I would think vacuum line would be cheap if you need a new length of line.

Does your engine RPM rise when applying the brake?  That's a test for air leakage.

What type SL do you have?

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both  tops
1994 E420

[dldubois]

Another potential cause could be the rubber brake lines to the calipers.   Over time they tend to swell which could lead to this result.  

I've owned two 280sls and have replaced on both.   One car had the brake lock up as a result and the other would engage the brakes.


Dana DuBois

1969 280sl auto
blue (906)
Corinth, TX

[Vince Canepa]

The valve is a simple check valve.  It is there to trap some vacuum in the booster should the engine stop.  If it is working properly, after shutting off the engine you should be able to feel it in the pedal if you apply the brakes several times.  The pedal will "harden" as you deplete the vacuum.  When my original booster failed the symptom was similar to yours.  The brake hoses are another good tip, althought they should fail more gradually.

Vince Canepa
1967 250SL
113.043-10-001543
568H Signal Red
116 Caviar MB-Tex