Wierd brake problem - No Brakes!

[RisQ]

Hi all,

I have been trying to solve this problem for a while now and its doing my head in 
I cant get the pedal to stay hard all the time so Its quite a wierd problem.

I'm clueless so need you guys help. here goes;

When I first got the car the rear brakes were seized and needed refurbing. I refurbed the rear callipers and obviously bled the brakes. After bleeding the brakes my brake pedal didn't come back although it was really low before the refurbishing. The Mk2 has a ABF in it with the Hydraulic pedal box conversion too. I changed the discs as they were pretty shot and the pads. In the bleeding process the fluid was really bad. It was very white and bubbly at first then it will turn into a dark green colour, plus it smelt like sh**. This obviously meant the car had been sitting for long. I bled the brakes till the new fluid was coming out but peddal stiffness never changed.

I decided to change the master cylinder as no matter who i spoke to said this. After swapping out the Master cylinder for a new one the problem still continues but its a wierd one now. When the car is stationary, If i pump the brakes, it feels normal. As soon as you go forward and push the pedal, you can feel the pedal getting loose and it goes staight to the floor. Funny enough, if you slap it into reverse and start going backwards, the pedal starts to stiffen each time you press it.

People suggested it may be the servo but after disconnecting the servo vacuum hose it still done the same thing but obviously the pedal was stiffer due to no brake assist. I done some of the test that I know on testing servos like pumping the brake pedal till hard when the car is off and starting the engine to see the pedal go down. I also pumped the brakes till really stiff when engine was off and left it over night, when i lossened the vacuum hose there was good vacuum. I also tested the one way valve on the servo and it looks to be good.

So no braking pedal when moving forward but pedal returns after reversing.

Sorry for the essay, really need to get this sorted as its p***ing me right off lol.

Thanks alot.

[oakgreengolf]

There's a valve on the rear axle this needs to be held open when bleeding the brakes, worth a check. On lowered dubs it's normally not adjusted and is permanently in the closed position. http://www.golfgtiforum.co.uk/index.php?topic=27064.0

[weazgti]

People also recommend raising the rear caliper up so the bleed nipple is pointing directly upwards.
I spent ages doing mine and ended up getting a eezi bleed that connects to spare tyre. Took 10 mins after a master cylinder change.

[RisQ]

Yep the car is on lowering springs so not sure if i have to bleed differently because of this but I bled the brakes when the bias valve was open. Also done the calliper nipple facing up too but no luck.

[jezza16v]

Sounds like a problem with the rear pressure limiter, as you reverse the back end lifts putting it back to the normal position....?

[RisQ]

Yep but funny enough, Even if you reverse really slowly making sure the car is not moving in a vertical motion, it still gets stiffer. So thats why I'm just lost.

[Thom89]

Sounds like the problem may be Vac related, do you have a knackered engine mount that is allowing too much movement on the engine? could be that the Vac tube to the servo is possibly fractured, and is opening up one way or the other when reversing because of excessive movement...

Are you confident the servo is not knackered?

Also, is there an inline valve in the middle (ish) Vac tube? is it the right way round? is it knackered?

Triple check that all flexi hoses are in perfect condition, sometimes the hose can collapse, and the fluid will find a new path up the side of the tube! you need to inspect carefully while someone puts the brake pedal under pressure

Check that all pistons in calipers are operating freely, and not getting hung up... check this, don't just assume they are OK!

Thom

[RisQ]

Thanks for the reply Thom.

I still think its vacuum related as when the car is off it doesn't act like this.

My engine mounts seem to be ok and the pipe has enough slack. TBH i don't even know how to fully check the one way valve. I only took the whole pipe out with the one way valve and submerged everything in the water. I blew from one side to see if there were any bubbles and it held pressure ok. The other side let out the air without a problem with no other bubbles from the pipe exept the end.
If there is another way to test please tell me, I would like to check it.

I dont know if the servo is faulty to be honest. I checked it by the methods above - pumping pedal till hard then starting car. Also pumping pedal till hard, leave over night, pull off the vacuum hose and listen for a hissing (vacuum leak).

The valve looks to be good but as its ABF converted the previous owner who did it made the vacuum hose from 3 different hoses. One hose from the servo to the valve and the same type of hose from the valve to the engine. The last hose has clamps from the manifold to the vacuum hose. That last hose seems to be good as if it wasn't the car would idle incorrectly.

I've checked all the hoses all the way from the MC and I can see no leaks. Ive also checked the callipers and they seem to be operating well.

I'm going to go to my local Mk2 parts guy and pick up and a used servo to check and completely wipe out of the list.

Thanks again for the help.

[Thom89]

The vac pipe is supposed to be ridgid but flexible hard plastic, is there any chance that the pipe is collapsing under its own vacuum? You should be able to suck air from the manifold end of the pipe, but nothing from the servo end...
Also check again that the calipers are tight, and no excessive movement on the sliders that could cause an angle change when rolling forwards or backwards... A lot like the problem you get with cheap push bike brakes! Where the bike would roll easily going forwards, but jam up going backwards if the makes sense

Thom