Vacuum Servo Setup

[steverrquattro]

My car has had a vacuum servo conversion. It's a UR quattro with an ADU engine.

Basically when I took it for an MOT last week having been off the road for 10 years, the brakes jammed on in the middle of a busy railway crossing. Lots of revs and the smell of a clutch overheating and it limped to the other side whilst I awaited the AA.

After around 30-45 minutes of waiting, the brakes ease off and you can drive it for another 10 minutes or so before the inevitable happens again.

So having listened to lots of advice I changed the master cylinder. However, I still have the same problem and TBH the old one looked fine with good pressure and no signs of leaking.

Something that I noticed which is puzzling me is that when I put the master cylinder on, the piston engages the rod and is compressed around 20 mm. I might also add that there is separate bolt of sorts (could be home made) that the servo rod engages, which then loosely sits inside the master cylinder.

Should the rod from the servo be setup so that it is almost touching the master cylinder?

[quattro-nick]

hi steve
there should always be a little play between the cylinder and the rod .

[steverrquattro]

hi steve
there should always be a little play between the cylinder and the rod .

Are you saying that the cylinder should not be compressed at all until the brake pedal is depressed?

[quattro-nick]

yes,that's correct.when you put the mastercilinder on the vacuumservo there should not be any resistance at all
if your brake pedal still stands in normal position it can be that the vacuum servo is broke

[Error404]

You want the servo pushrod to be almost touching the master cylinder piston shaft in the rest position.

[steverrquattro]

You want the servo pushrod to be almost touching the master cylinder piston shaft in the rest position.

That's what I thought. Does the rod adjust from inside the car (under the dash), or can you do it from the engine bay side by turning the rod?

[Error404]

You can adjust the servo pushrod length by screwing/unscrewing the clevis at the pedal

[AndyP]

On a railway crossing??

I can only imagine the fear and panic that you experienced!!

[AndyP]

Let me know if you want to come and look at mine for reference

[steverrquattro]

On a railway crossing??

I can only imagine the fear and panic that you experienced!!

For real, you couldn't make it up and the funny thing was, I actually looked left and then right to see if a train was coming. It was like a scene out of Final Destination.

I'm going to have another go at it tomorrow. If all goes well, I'll pop over soon.

[Error404]

I'm going to have another go at it tomorrow.

Don't push your luck mate

[steverrquattro]

Just doing a bit of research and came across this........so it looks like I'll be taking it all apart again tomorrow and putting the old master cylinder back in but this time with the push rod just shy of the mater cylinder.

Check Booster Pushrod Adjustment


The pushrod that actuates the master cylinder must be properly adjusted. Ideally there should be only slight clearance between the booster pushrod and the master cylinder pushrod. Interference will preload the master cylinder. When the system is preloaded, it builds pressure each time the pedal is pressed. Since the master cylinder is not allowed to fully release the pressure from the previous stroke, the system will eventually lock the wheels. Too much clearance will cause excessive freeplay in the pedal. In almost every case, if you bought your booster and master cylinder from us, we pre-adjust the booster pushrod and master cylinder for you so this is not a problem.

Just to finish off the thread, as so many are often left without giving the conclusion, I took a few measurements and drilled out the bolt to allow the rod from the vacuum servo to penetrate further. When the master cylinder was then put back on, it was not compressed.

Then thanks to the help of my assistant (MAC) the brakes were bled and we went for a spin.

Looks like I have a boost leak now:. Not a problem though as some of the pipe I have used is only 2 ply and apparently should be at least 4. Also, the small vacuum hoses look original, so they all need replacing and this is cheap and readily available from GSF.

Thanks again for everyone's help.

[mac2226]

No problem Steve glad to give you a hand on swb any time, yes that top silicone hose needs changing mate asap