Wierd brake problem

[AMck]

Hi, I am pretty new to Mercedes ownership having just bought a 2000 SL320.

I'm pretty sure there is a problem with the brakes - possibly the servo and would be grateful for comments.

I also have a slight pulsing on the brakes, but I am sure this is a warped disc, both the front and rear discs are close to their limit and the rear pads are well worn.

If I slow down with reasonable but steady pressure on the brake pedal all seems well, but if I then need to quickly apply more force the pedal seems to go hard and I have to press very hard to get any extra braking.
Also when initially pulling off my drive after starting the car if I apply the brakes they seem hard and pretty ineffective taking almost all my effort to stop the car moving when in drive and without my foot on the accelerator, but as the engine is cold it is at fast idle.

I have put my foot on the pedal when starting the car and it goes down as the servo kicks in, but if I rev the engine it pushes the pedal back up against my foot. I have tried to listen for any leaks under the bonnet but can't hear anything. In the car if I keep my foot on the brakes I can hear a constant hissing from around the pedal area.
Also after turning the engine off it only takes 1 press of the pedal to exhaust all the vacuum and lose any assistance.

Does this sound like a possible leak in the servo diaphragm? If it is is it possible to strip and repair the servo or will it need a new one? Does anyone know of a repair kit or the part numbers of a replacement servo?

Thanks

 

[television]

Do check the one way volve on the pipe from the inlet manifold to the booster 70 in the link

http://www.detali.ru/cat/oem_mb2.as...GA=722.607&CT=F&cat=44F&SID=42&SGR=060&SGN=03

 

[AMck]

Thanks Malcolm,

But when I look at the diagrams I can only see a 70 in the diagram for a LHD car which is listed as a line. My vac pipes look more like those in the 2nd diagram. Should I be looking for a one way valve in pipe 80?

 

[television]

Yes there has to be a one way valve in that line.

Servos in general work so that when the tank is at max vacuum you have about 8 pumps of the brake pedal to empty it. If the tank was empty, when you pushed the pedal the idle would fluctuate.This is where the one way vlve comes in, and stops the vacuum stored from going back to the engine. When you start the engine and have your foot on the brake pedal, the pedal should sink as the servo cuts in, if you turn off,. and push the brake, the pedal should slowly get harder and harder with each push till the tank is empty

Do not overlook the BAS assist control unit

 

[turbopete]

regarding your brake vibration, ive worked on a few with this problem now and the wearing face on the inner side of the rear discs corrodes and then lumps of corrosion wear off leaving the discs pitted, hence the pedal vibration. however i seems that you are probably replacing them anyhow. justthought id mention it for anyone who may need to know for future reference

 

[AMck]

Malcolm - I have had an initial look and I think I have identified the valve - but not easy to get to - it appears to be tucked away under the cabin air filter box.

To check I disconnected the pipe from the servo - there was a strong vacuum while the engine was idling, but as soon as I switched off the engine the vacuum immediately went. I know it is only a short piece of pipe from the servo to the 1 way valve, but I would have thought I should have still been able to feel some vacuum.

After switching off the engine the tank empties in 1 press of the pedal so that there is no assistance the 2nd time you press the pedal.
Also I noticed that if you keep your foot on the brake the idle seems to become a little lumpy and there is a constant hissing coming from the brake pedal area whilst the pedal is pressed.

TurboPete - I think you have hit the nail on the head with your diagnosis - Before buying I looked at the new MOT's advisory notes - it identified the rear brakes as the problem and having a look at them the outer surface is quite worn, but the inner surface much less so.

 

[AMck]

I have just spoken to the dealers and they have quoted part no - 1294303029 as the pipe with the one way valve - which relates to no 75 in the diagram - looks like I was looking in the wrong place.

I did disconnect the pipe at the point in the diagram just above no 90 and was able to both blow and suck down the pipe to the engine with no resistance.

Will have to go take another look.

Just hope it's not the servo as that is £626 + VAT

 

[turbopete]

well every merc our family has owned has had that problem at some point, as have a few others i know! a preventative tip given to us by MB EUROPE was to find a quiet piece of straight, camera free (if thats possible nowadays!) road, every couple of months or so if possible, get up to 100mph or so, then brake as hard as possible, down to 20 or 30ish. as though you were coming off the motorway and onto a roundabout situation. do it a couple of times if possible. this generates enough heat in the rear brakes to dry them out and prevent the corrosion. so we were told anyhow!

 

[wireman]

To me it sounds as if your servo's control valve is at fault.
In normal operation some air is drawn into the servo via a small filter/strainer mounted concentricaly with the operating rod (which pokes into the car). If the control valve (mounted inside the servo on the diaphragm assembly)) is faulty there could be a direct flow of air into the manifold whilst the pedal is pressed, it is here where I suspect the trouble lies since the hissing you describe suggests continuous flow which should not occur under any curcumstances, there should only be a little air admited and only for a short time.
Continuous air would weaken the fuel air mixture sufficiently to cause poor engine running or even stalling at tickover.

To verify the non return valves fuctioning try pressing the brake pedal an hour or two after the engine has been stopped, if the pedal is rock hard the vacuum has been lost whilst the engine was stopped, if you get a rock hard pedal try leaving only a minute or so before you check, this might give you an idea of how quickly it is leaking off.

If the non return valve is very leaky it is possible (particularly with carburettor engines) for the servo to draw in petrol vapour as the engine stops the condensate of petrol inside the servo will destroy your servo's diaphragm.

 

[AMck]

OK have tested to see how long before vacuum goes - it was gone after 30 seconds. A faulty one way valve could explain this, but am I correct in thinking this wouldn't explain the constant air hissing and entering the boost unit from the brake pedal area?

Is it possible to have the servo unit rebuilt, or must it be replaced as a complete unit? If as a complete unit would it be reasonable to fit a used part as I'm guessing this isn't a particularly common fault.

I only bought this car 2 weeks ago - so far have this brake problem and went to have the A/C regassed to be told the condenser is shot - I think I might need to go back to the small trader I bought it from even though there was no warranty I think I might still have some legal protection.

Thanks for all the help to this newbie to date - hopefully as I get to know the car I will be able to put something back.

 

[television]

OK have tested to see how long before vacuum goes - it was gone after 30 seconds. A faulty one way valve could explain this, but am I correct in thinking this wouldn't explain the constant air hissing and entering the boost unit from the brake pedal area?

Is it possible to have the servo unit rebuilt, or must it be replaced as a complete unit? If as a complete unit would it be reasonable to fit a used part as I'm guessing this isn't a particularly common fault.

I only bought this car 2 weeks ago - so far have this brake problem and went to have the A/C regassed to be told the condenser is shot - I think I might need to go back to the small trader I bought it from even though there was no warranty I think I might still have some legal protection.

Thanks for all the help to this newbie to date - hopefully as I get to know the car I will be able to put something back.

You do have re dress from any trader, this is a dangerous situation, all cars sold by a dealer must be fit to drive

 

[Number_Cruncher]

>>there is a constant hissing coming from the brake pedal area

I agree with wireman - for a suspended vacuum servo, this should never happen. It's either the valve which isolates the two servo chambers upon the initial application of pedal force that's failed, or, it could even be a punctured diaphragm.

The OP needs to have a new servo fitted.

 

[AMck]

Thanks guys for all the help.

I have written to the trader and will await his response, meanwhile the car will have to remain on the drive

 

[Gangley]

The hard braking tip is sound. I had mine on a rolling road on the annual test and the tester had to keep hammering the brakes to get the rust off the rear disc on my e class. It eventually did the trick. Not enough regular hard braking was the problem. Granny!

 

[124coupe]

Your rights when buying from a dealer:

When buying from a dealer, the law says a car must be:

*Of satisfactory quality. It must meet the standard a reasonable person would regard as acceptable, bearing in mind the way it was described, how much it cost, the age and the mileage on the vehicle. Amongst other things, this covers the appearance and finish of the car, its safety and its durability. The car must be free from defects, except when they were pointed out to you by the seller.

*As described. If the advert states “air conditioning and CD player” then the car should come with these features and they should be working.

*Reasonably fit for any normal purpose. It should get you from A to B.

*Reasonably fit for any other purpose you specify to the seller. For example, towing a caravan.

Clearly the brakes fall under point 1&3 and the A/C under points 1&2 (unless the dealer described it as not working?)