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Brake booster issue


Macker
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Hello everyone, i'm almost finished rebuilding a series 3 on a new frame, lhd conversion, 200tdi, etc. Before the rebuild the truck would stop fine after double-pumping the pedal. The master was leaking so it got replaced. Now here's my issue: even when the truck is off, I can pump the pedal and it will go straight to the floor. I can pump it repeatedly and quickly to build up pressure but as I keep pressure on the pedal, the air escapes somewhere. I'm wondering if I have a bad brake booster. I pulled the hose off the booster and when the truck is idling it has good vacuum. I'm guessing I have a leak in the booster diaphram? Any help would be great. Thanks,

Mack

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Sounds like you've got air in the hydraulics which is why you have to pump it to get a hard pedal. The pedal slowly going down suggests a fluid leak if you're loosing fluid or fluid going past the master cylinder seals if you're not loosing fluid.

It won't be a servo fault.

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I have no fluid leaks. Should have mentioned that all the lines/hoses/junctions from the master to the wheels have been replaced, and now the master is replaced also. I was thinking about using a pressurized bleeder system next. When I press down on the pedal and it slowly goes to the floor, I can hear noise coming from the diaphragm that the booster air hose is connected to. (the engine end of the hose) Should I be able to hear noise coming from there? to me it seems that's where my leak is, but I'm not sure

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Extra pump? no extra pump, just manifold vacuum. I'd agree that the servo should hold vacuum if it were leak free, but it seems it won't hold vacuum for more than a split second, unless of corse it's in the hydraulics. I was sure what it was called, but this little number is where I hear the "noise" coming from when I push the pedal down and it goes to the floor. In the mean time i'll be re-bleeding the brakes with my vacuum assisted bleeder.

post-20911-089120000 1283120610_thumb.jpg

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As a matter of interest, do you you have the single or twin leading shoe brakes on the front, single or split system, and if split is the PDWA fitted?

The only way a servo can cause the pedal to go to the floor is if the pushrod between it and the master cylinder becomes displaced.

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It's a twin system. No PDWA, it has a T for the front 2 brakes. It actually has fj40 axles with disc front, and a proportioning valve for the rear drums. Sounds like it's not the booster. I vacuum bleed the brakes, then pressure bleed and still it's the same. It's spongy and occasionally it will build slight pressure after repeated pumping, but it feels like there's still air in the system. Not sure where though...

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Are the bleed screws at the highest point on the calipers/slaves?

Have you tried isolating parts of the system with hose clamps to see if you can isolate where the problem is?

I assume the servo is the type directly operated by the pedal, and not a remote one?

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You have a vacuum pump - its that thing where the injection pump would be on a 2.25. (S3 2.5 diesel have a butterfly valve in the inlet which provides the vacuum when the throttle is shut. They also have a tank reservoir.)

The servo itself should hold some vacuum - generally enough for 1 - 3 brake applications. I wonder if there should be a non-return valve somewhere and its been ommitted on the conversion.

However even with no vacuum you should have a firm pedal - even if you're not strong enough to apply the brakes.

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Are the bleed screws at the highest point on the calipers/slaves?

Have you tried isolating parts of the system with hose clamps to see if you can isolate where the problem is?

I assume the servo is the type directly operated by the pedal, and not a remote one?

I don't believe i'm able to relocate the bleed screws, they're sort of in the middle of the drums. I haven't clamped off any lines, what's a safe way to clamp off the line without damaging it? And the servo is the origional series 3 servo, so it's directly connected to the pedal.

You have a vacuum pump - its that thing where the injection pump would be on a 2.25. (S3 2.5 diesel have a butterfly valve in the inlet which provides the vacuum when the throttle is shut. They also have a tank reservoir.)

The servo itself should hold some vacuum - generally enough for 1 - 3 brake applications. I wonder if there should be a non-return valve somewhere and its been ommitted on the conversion.

However even with no vacuum you should have a firm pedal - even if you're not strong enough to apply the brakes.

I was confused, so it is a pump rather than straight manifold vacuum.. But I'm wondering if you may be right about an omitted non-return valve. When I push the pedal down it starts to be firm and then the pressure seems to leave somehow. it feels like the pressure the booster creates is being leaked into the pump, but maybe only because the engine isn't running perhaps? It seems that even if the engine wasn't started, it should be able to hold the pressure, even if it is difficult to press down. As the pedal slowly goes farther down, I can hear noise coming from the pump. Is this normal anyone know?

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There are special hose clamps with rounded "jaws" so they won't pinch the rubber pipe. Not very expensive and a very useful tool.

The connection between the pedal and master cylinder is a solid bar - the servo just increases the force in the pedal.

As I mentioned above there is no way a leaking servo can cause a soft pedal.

If the bleed screws aren't at the highest point that could be why you're not getting the air out.

One method I use is to clamp the pipes off that you're not bleeding, attach a length of rubber pipe to the nipple and immerse the end in a jar of brake fluid. Open the nipple fully and press the pedal hard and fast, that should cause turbulence in the slave, thus scooping the air out.

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