Brake assist failed - do brake booster die?

[dingram]

I was heading out last night in my Golf (IV, GL Rally, 1.6l manual) and the brakes were fine heading down the street (and were good on the drive home 30min before). At the next intersection the brakes were heavy, just like trying to drive while the engine is off.

I've checked that the vacuum line is still in place between the booster and the manifold, and the brake fluid levels are good. The fluid was replaced just over a month ago as part of a regular service.

Do brake boosters go bad all of a sudden? Any hints on things to check before I empty my wallet over a service place? I'm looking for someone close to Indooroopilly (Brisbane) to do the work since I'm not keen driving all over with dodgy brakes.


Cheers,
Dave.

[VDUB-BUD]

i'm currently experiencing similar problem. I bled brakes a while ago and still doing it. I might completely change fluid n try again.

Its bad when your going down a hill n this happens, its disturbing when you have no confidence in your braking system.

[Blitzen]

Yes, the booster can fail. You may have a split in the rubber inside the booster.

[jamesdavid]

Hey Dave,

Boosters are simply a mechanical component, essentially there are 2 chambers, one a cabin pressure (atmospheric) the other in vacuum (supplied from intake manifold usually, or a diesel your exhauster pump on the back of the camshafts as diesels produce little vacuum due to engine stroke compared to petrol engines. All that separates is a a rubber diaphragm, so if this has a hole, obviously we have no pressure difference and pedal will be very stiff.

Brake fluid concerns will not create a hard pedal, air in a hydraulic system cannot be compressed and will create a spongy pedal, it will seem normal at first but when the air pocket comes the pedal will drop.

Pump up the pedal with the engine off so its stiff. then start the car, if the pedal remains hard you have a problem with your booster and most likely will have a hole in the diaphragm as you are not getting any pressure difference. A correctly operating booster should sink at this point as a large difference in pressures will be created when the booster is supplied vacuum from the exhauster pump or inlet manifold.

Also if its petrol, take off the line from the inlet manifold to the booster and block it off with your finger, if you can feel vacuum, like your finger is being sucked in then you can assume your vacuum supply is ok.

Hope that helps, James

[Guest001]

"Brake fluid concerns will not create a hard pedal, air in a hydraulic system cannot be compressed and will create a spongy pedal, it will seem normal at first but when the air pocket comes the pedal will drop." quote


Dont you mean that air in a system CAN BE COMPRESSED and cause a spongy pedal by the fact that the air is compressing and causing the pedal to drop.

Air or any gas can be compressed whereas a fluid cannot normally be done so.

To wit getting water in a cylinder. Causes havoc because it remains a virtual solid

[jamesdavid]

Sorry lol yes, i was in between reading measured values and typing, that will learn me not to proof read haha.

[brad]

also check for the vacuum line collapsing internally (therefore no vacuum to booster) or non-return valve faulty.

[h100vw]

I have seen a splits in the hard lines on a more than one mk4, my S3, old A3 and A4. Where they are shoved onto a joint, the elbow into the booster, onto non return valves etc. The hard line often splits if there has been any sideways tension applied.

If the pedal is rock hard but you don't appear to get much braking, its an assistance problem for sure. If the pedal goes down a long way then air would be the most likely cause.

Before pulling the booster, double check the vacuum lines and pay attention to all the joints.

Gavin

[dingram]

If the pedal is rock hard but you don't appear to get much braking, its an assistance problem for sure. If the pedal goes down a long way then air would be the most likely cause.

Before pulling the booster, double check the vacuum lines and pay attention to all the joints.

Gavin

Yep, the pedal is hard, but I am able to stop the car. I'll pull the hose off and see if there is vacuum on there (as someone else suggested), as that will test the rigid hose for leaks too.

Would a failure be like flipping a switch (good one instant, bad the next), or more likely to be a gradual fail? Prices for a replacement are pucker inducing, so hoping it isn't the booster.

[dingram]

'Good' news: poking around, preparing to remove the hose from the brake booster and the hose just fell off the elbow. The plastic has split. There isn't much room to extend it, so I think I need a new rigid hose section rather than a brake booster. Heck, I need a new hose regardless. I guess if I had a manual vacuum pump I could check the booster that way.

Time to find out the parts price.

The workshop I go to recommended heatshrinking the connection on as a bandaid, so I'll give that ago so I can get to the workshop safely.

Unfortunately there isn't much room, and the heatshrink didn't work. I was able to make a petal out of the end of the hose and jam it on a little harder, and that has given some brake assist. If I pull the hose I get a hissing sound, so upstream seems in order. At least I can be a little happier driving to the workshop. I think the car is telling me it wants to retire (radiator temp sensor, castor bushes, front brakes all needed fixing in the last month).

The split was on the firewall side, so hard to see. @VDUB-BUD you might want to have a poke around at the hose connection on your brake booster to feel for a split.