Brakes. Vac assist only when accelerating?

[Darknights]

Hi all, I've got a serious pain in the **** issue with my 2001 Audi A4 Avant sport 1.8t, that's making it unsafe to drive and wanted to pick your brains.

Basically I only get vac assistance on brakes when accelerating? Which is v odd cause it's a 1.8t, so should have no vac when on boost right?

I've replaced all the 1 way valves (4x of them), the suction jet pump, PCV pancake valve and this morning done master cylinder & servo and bled brakes, but to no avail? Got any ideas as I'm stumped and wanna sell the car in a couple of weeks but can't if brakes aren't up to scratch?

Just done fault code scan but nothing :-(

I'm definitely getting vacuum to servo, but only getting vac assist when your accelerating not when off boost?

HELP!!!! :-(

 

[Darknights]

Here's the parts I've replaced

 

[jbh]

have you checked the servo its self to see if theres any rust on the underside? Ive had to replace one on a passat a while ago.

 

[Darknights]

have you checked the servo its self to see if theres any rust on the underside? Ive had to replace one on a passat a while ago.

Yeah no rust on the one I took out or the new (second hand one) I put in. Don't think it's that as I get full vac assisted brakes when accelerating? Brakes work, just only when they're not supposed too!? I'm confused.

 

[Darknights]

What I don't understand is that on a turbo car, when accelerating you shouldn't in theory get vac assist as they only run at vacuum when your foot is off the gas? Mine seems to only get vacuum when on boost, which is the wrong way round?

 

[aragorn]

Even on an NA car, you'd get no brake boost when accellerating (at or near full throttle anyway) because theres no vacuum in the manifold.

What they do, is use the servo casing as a reservoir with a check valve in the pipework, which stores the vacuum. If you run the engine at idle for 30 seconds, shut it off and wait 30 seconds, then pull the hose off the servo connection at the bulkhead, you should get a hissing noise as the vacuum is released.

Can you stick up a photo of the engine bay showing the pipework?

 

[Darknights]

Heres a couple of images to show which bits have been changed, i'll try and get a few pics of the actual bay soon.

Gonna check that i put all the new 1 way valves in the correct directions.

 

[Alex C]

You haven't somehow connected your pcv system to your brake servo have you?

 

[Darknights]

You haven't somehow connected your pcv system to your brake servo have you?

Possibly? Gonna double check all the vac pipes and hopefully get it sorted. So frustrating replacing nearly everything and only working when accelerating :-(

 

[Alex C]

that's my only suggestion. If you were feeding boost to your brake servo then you would have a rock hard brake pedal. The only vacuum source I can think of when you are hard on the throttle is the TIP. The TIP is a vacuum source to your PCV system under acceleration/boost

 

[aragorn]

This is why i asked for pics of the engine/pipework. Much easier to just look at whats going on.

 

[Darknights]

This is why i asked for pics of the engine/pipework. Much easier to just look at whats going on.

Yeah unfortunately it's dark when I get back from work. I'll try and grab a shot in the morning if I get a chance

 

[Darknights]

Found a few pics of when I fitted the valves

 

[aragorn]

Hmm weird!

On most A4 1.8T's that servo line on the bulkhead just goes straight to the manifold (with a check valve). Yours has some extra parts presumably becuase its a late model car, and unfortunately i've no idea what they do.

Maybe theres a broken pipe causing the vacuum to leak away?

 

[Darknights]

i've given up now, taking it to a freinds garage to have a look :-(

 

[Broken Byzan]

Probably faulty suction jet pump. Bin it off and just use a one way valve

 

[Darknights]

Just replaced suction jet pump so don't reckon it's that?....