BRAKING/VACUUM

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bowser107
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Hi, having problems with my brakes.

Sometimes pedal is 'long' other times solid when first pressed. Also if pressed three times it goes solid.

Could it be the vacuum pump ? If so is there a few checks to do, caN it be rebuilt or is the only option new. Many thanks

MODEL 300D  REG D648CPV  CHASSIS WDB4603322705221

stig
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Re: BRAKING/VACUUM

If when the pedal is working at the top, and the brakes work strongly, then the vacuum is ok, you can check this by turning the engine off (when stationary!), pumping the brakes and the vacuum will eventually diminish, you'll feel it. If there was no vacuum, you'd have trouble stopping at all, and the vacuum shut off on the engine wouldn't work, if that's what you have.

Your problem is more likely the rear brakes way out of adjustment, seized auto adjuster maybe, and you need to 'pump' the brakes before the shoes are against the drum giving normal feel. Check that first, it's the most simple answer.

John757
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Re: BRAKING/VACUUM

Could be air in the brakes... Bleeding them will help.. Brake fluid is hydroscopic

and over time  can retain moisture reducing brake performance .  Replacing all the fluid every 2-3 years is recommended .

This will often cure a spongy brake pedal 

 

Regards

 

John 

 

 

 

Pistonhead
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Re: BRAKING/VACUUM

Please excuse a correction of terms John,

You meant Hy-gro-scopic which means substance absorbing moisture from the atmosphere; Hy-dro-scopic is a term used for a device to view under water.

To be more precise; Yes you are right in mentioning brake fluid is hygroscopic but this action on brake fluid does not introduce air bubbles as such into the brake fluid.

The action of moisture over time in brake fluid affects the overall boiling point of brake fluid and when this condition is apparent the degraded brake fluid under prolonged braking tends to boil-over due to its resistance to withstand high temperatures generated by braking diminishes. The heat braking generates radiates into the brake fluid and it then starts to boil. When boiling occurs it is then, the fluid breaks down to induce air bubbles and when air bubbles are present the compression forces applied for braking reduces as air can be compressed and fluid can not. Hence the soft pedal or diminishing braking effort and effect.

Having mentioned old brake fluid has reduced efficiency under prolonged braking; I have experienced first hand soft brake pedal several times on different vehicles, both Mercedes models,  applying the very first depression after overnight parking under cold conditions.  Renewing the brake fluid solved my problems on both occasions. So from that it could mean as air bubbles develop  initially under prolonged braking, conditions could develop so after when brake fluid has cooled down the air bubble could build up and not dissipate.

Sermon over. 

Pistonhead
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Re: BRAKING/VACUUM

Re: Browser's comment; if the pedal becomes solid after three applications, this suggests initially possibility of air in your system, renew the brake fluid first and lets take it from there. There are other symptoms but taking a step at a time go for the brake fluid renewal first and report back.