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300 tdi brake servo and vaccum pump testing


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#1 Ryan

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Posted 13 January 2008 - 01:03 PM

I'm just about to do some testing to trace a brake fault (the pedal needs one pump before I get a firm pedal - I have already replace the brake fluid and bled the system) and will be testing the servo as per the instructions in my Haynes manual. The query I have is this (which may be a bit long winded):

1. There is no test given for the vacuum pump. How much "sucking" does it do? If I remove a pipe and stick my thumb over the pump connector, should the amount of suction be noticeable? i.e. will my thumb stick to it?

2. Is the test on the servo (which involves purging the vacuum, keeping the pedal pressed while the engine is started etc) only going to show up a fault in the servo, or could something else cause the same symptoms i.e. if the vacuum pump is not working correctly?

3. Are there any checks I should do before checking the servo to eliminate them first?

By the way, it does not have ABS.

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Ryan
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#2 Ryan

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Posted 13 January 2008 - 01:46 PM

Well, I couldn't get the check valve out of the servo, and I couldn't get the pipe off the pump. The servo appears to be operating correctly as per the Haynes test.

Any ideas on why I'm getting the excessive pedal travel initially? For what it's worth, the rear pads were replaced about 1-2 months ago with Mintex, and the front ones appear to have quite a bit of material left when I checked them yesterday while bleeding the brakes.

Am I going to be spending some money on a new master cylinder?
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#3 Comical Engineer

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Posted 13 January 2008 - 01:57 PM

You should be able to feel the suction from the vacuum pump very easily.

That said, if the vac pump was not working, you would probably have to press the pedal very hard to stop the beast if it was not giving a decent vacuum. These pumps tend to be pretty reliable.

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#4 Les Henson

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Posted 13 January 2008 - 01:58 PM

If you have to press once and then again to get a good brake, then there's a possibility that there's air in the system. To test the servo - pump the pedal until it's rock solid with the engine switched off, and then while you maintain pressure, start the engine and the pedal should sink about an inch and then maintain that position - if it creeps down, then suspect master cylinder allowing air into the sytem through a faulty seal. To get the valve out of the servo you need to squirt WD40 onto the rubber and work it around to get the fluid around the rubber - it should then lever out. If there's a hiss as the valve comes away, then you can assume that the servo is ok. You have calipers all round? then it's unlikely to be a problem with these as they almost never leak. Jack up each wheel in turn and check for play in any of the wheel bearing/swivel pins - wear in these components causes the disc to oscillate, which pushes the pad back too far and creates a gap, which requires one pump to close and then another to apply the brakes. Leaks on the master cylinder are either where the master is bolted to the face of the servo, or internally where it runs down the pedal and onto the floor.


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#5 Ryan

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Posted 13 January 2008 - 04:22 PM

Ok Les, thanks. Could be the master cylinder then. Unfortunately the manual didn't specify how far the pedal should drop as you start the engine. My pedal was sinking for about 5 seconds (probably about half way down) before it stopped. I'll try and have another go at the valve tomorrow morning. And I'll check the master to servo joint to make sure that's nice and tight.
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#6 TheRecklessEngineer

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Posted 13 January 2008 - 08:14 PM

My guess is the master cylinder. Just replaced the vacuum pump on mine...still giving a perfectly good vacuum, but making one hell of a racket! I'm guessing you would hear it before it gives out.

Change the master cylinder - even if it is not the fault, it's probably not 100% if original.

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#7 RRX

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Posted 13 January 2008 - 08:54 PM

2 pumps to get brakes i would say master cylinder also

#8 Ryan

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Posted 13 January 2008 - 09:22 PM

Thanks all. Getting some prices from the usual suspects - new one seems to be about 100 plus the dredded. If I don't go main stealer, are there any brands to steer clear of, or which are "damned good value?" I'm hoping that any Bearmach do should be ok. Trouble is, most of the usual suspects don't tell you what brand they are selling.

Edited to add: Just to be clear about the symptoms, I do get brakes on the first pump, but it's firmer (ooer ;)) on the second pump. Is the consensus still the master cylinder?
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#9 Les Henson

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Posted 13 January 2008 - 09:28 PM

If you can get a repair kit, then another option is to rebuild the master if that turns out to be the problem.


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#10 Ryan

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Posted 13 January 2008 - 09:31 PM

If you can get a repair kit, then another option is to rebuild the master if that turns out to be the problem.


Les.



Err, ooh, ummm. Mechanical numpty... :blink:
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#11 Les Henson

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Posted 13 January 2008 - 09:38 PM

Not for much longer!

http://forums.lr4x4....?showtopic=5300

Different master, but the method is the same.


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#12 Ryan

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Posted 13 January 2008 - 09:48 PM

Problem I've got is this car is my only car. If I get the cylinder apart and find it's knackered (I believe that is the technical term!), I'm stuck like a stuck thing.
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#13 Les Henson

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Posted 13 January 2008 - 10:11 PM

As soon as the internals are taken out you only need to check for scoring/steps in the bore of the main body. If they are, you can put it back together on a temporary basis while you get a whole new cyinder. You could remove, inspect, and replace the cylinder in a couple or so hours.
It is very rare for the bore of a cast iron cylinder to be scored nowadays. The piston assembly is usually alloy, so is usually what suffers if there's any metal damage. Seal failure is generally the only problem in my experience.


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#14 Ryan

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Posted 13 January 2008 - 10:19 PM

Okey Dokey Les, if you say so :D

I might have to blag some tools tomorrow.
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#15 Ryan

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Posted 13 January 2008 - 10:22 PM

Actually, just had another thought...I'll need to blag someone's right foot to bleed the brakes again!

HobNobs anyone? ;)
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#16 Les Henson

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Posted 13 January 2008 - 10:32 PM

75 miles for a couple of biccies - I don't think so! :D


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#17 Ryan

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Posted 14 January 2008 - 10:49 AM

I've just had another thought (I'm sorry :rolleyes: ).

When I was bleeding the brakes, I noted that the reservoir was split into two sections (front and rear (nearest the servo)). As I bled the brakes, only the rear compartment was draining of fluid. Does this point to anything else? Could it just be a blockage between the reservoir and the cylinder body which means the secondary piston is not getting any fluid?
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#18 Les Henson

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Posted 14 January 2008 - 11:08 AM

The chamber is split into two compartments so that if you have total fluid loss at (say) the rear, the front brakes will still work as they have what is in effect their own seperate reservoirs.


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#19 Ryan

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Posted 14 January 2008 - 11:30 AM

The chamber is split into two compartments so that if you have total fluid loss at (say) the rear, the front brakes will still work as they have what is in effect their own seperate reservoirs.


Les.


Thanks Les,

Ryan (still trying to get out of actually doing something this morning).
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#20 Ryan

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Posted 14 January 2008 - 01:34 PM

OK. The vacuum pump is ok and the check valve on the servo also appears ok. I managed to get the vacuum pipe off the pump but still couldn't get the check valve out. However, I gave the pipe a good suck and blow :ph34r:, and I could only suck, which I believe is a good thing...started the engine and I was definitely getting sucking at the pump. All put back together now and just off to get a cylinder overhaul kit.
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