THE brake story...continued...HELP!

[imported_brackie]

OK, so I got all of the air out of the brakes. Pedal is nice and hard. Then I start the motor. Pedal goes to the floor. Again, and again and again. Servo was fine before replacing the master cylinder. Switch of the motor and I've got brakes again. Any ideas??

[Volksfolks]

How did ya go bud, got me thinking hey,

Andy

[imported_brackie]

No progress.

Busy day today so not time to look at the car. Well...I looked at it but that was all! Got a funny feeling that it could be either air trapped in the secondary piston (diagonally split braking system) or the master cylinder that I got had some kind of insert missing from it's push rod. The only way to check the latter I guess is to remove the master cylinder and compare it with the old one.

[Golf Loon]

I`d bleed the brakes before taking it apart again.

[imported_brackie]

Not again!

I've got clean, air-free fluid coming from the bleed nipples (used different coloured fluid so I would know when it was thru') so I'm certain there's no air in the system...But wait! What do you reckon about the pressure regulator to the rear brakes? Could there be air trapped in there? If so, how do you get it out? There's no bleed nipple.

[Golf Loon]

I think the regulator is too simple for that. It just adjusts braking proportions dependant on load. As long as the angle of the car wasnt changed, sholud be unaffected. I`d put a bottle of fluid through the system to be sure its all bled up. Other than that, your taking the master cylinder apart again

[imported_brackie]

Bleedin' sick of this!

So I bled them again...... Lovely hard pedal with just he right amount of travel. Then I started the motor. Pedal to the floor. So it looks like the master cylinder comes off tomorrow and then what???

[Golf Loon]

Sounds like a bad booster

[imported_brackie]

Master cylinder woes

So I took off the new master cylinder to compare it with the old. Appears the same on the outside, so I looked at the (what I thought was) the push rod. Not a push rod but the end of the primary piston. The inside of the new one was drilled out at the same diameter all of the way down. The old one has a step in it about 2/3 of the way down. "Aha!" I thought.... "Maybe the rod inside the servo is the same diameter as the smaller diameter inside the old piston but smaller than the diameter inside the new piston, giving a longer length of travel." Nope. It fits down both. OK. So I take the primary piston out of both master cylinders and I find that they are of different bores! The old one is 12mm and the new one 9mm (or as close as I can measure with callipers...Wouldn't a vernier be nice?). I'm convinced that the booster (I call it a "servo") is OK. Where to next?

[Beige76]

Can you take the booster apart? How do you remove the primary piston?

[imported_brackie]

Booster

You can remove the booster, but nope you can't dismantle it. The booster is a sealed, non-servicable item. (Not that I think there's anything wrong with it, anyway.) But I'm prepared to be wrong, How can it be tested? And what could go wrong internally that would cause this problem? It's holding vacuum and when I'm brave enough to test drive it (you should see the roads here!) it's definitely working. The primary piston is easy to remove (circlip) and the secondary is held back with a stop-bolt.

[Sharkey]

Don't know a bunch about the Audi servos/braking system, but here's my few views:

Remember that the servo is simply an assisting device, the push rod of the MC is still mechanically connected to the pedal mechanism, so in theory a hard pedal with the engine off should remain hard with the engine on. Obviously, you've disproven this.

It might be time to fall back to more extreme troubleshooting. I'd think about getting some 10mm bubble flare plugs and remove the brake lines to the wheel calipers one-at-a-time, replacing the lines with the plugs, and trying it with the engine running. If your pedal still plunges to the floor with all the ports plugged, you can bet it's a MC problem. If you get a hard pedal with just one (or more) of the ports plugged, at least you know where to start looking.

Why servo on/off makes a difference is a mystery, but it's just not possible that your depressing the pedal with the vacuum present can allow the push rod into the MC to not be pressed also.

You do have a Bentley service manual for this vehicle, yes? My A2 Bentley states that "vechiles equipped with the rear load-sensing regulator valve must have the load sensing arm held down towards the axle" while bleeding. Is it possible that the ride height of the car changes when the engine is running, making the regulator change the volume or pressure of the brake fluid going to the rear axle? Just freewheeling thoughts here...

[imported_brackie]

Bentley

Thanks, Sharkey.. We don't get the Bentley manuals here. The one that I have is for the Rabbit diesel and I got that from the US
The Haynes makes no mention of depressing the lever. When bleeding the brakes I had to jack the car in order to get under (I ain't a kid anymore!) so maybe you're right and I need to have it depressed or on the flat. Excellent suggestion about making the blanks. I assume that these go into the MC ports? (The Audi has twin circuit braking on diagonally opposite wheels.)
Will give the "depressed lever" trick a go today.
Great to have you on the board. Call in regularly and add to our knowledge base!

[Sharkey]

Can't get Bentley manuals??? Have you tried http://www.rb.com ?? I find used bentley manuals in the used book stores here for about $12-18US a pop. My A2 manual (which I bought expressly for installing an ABS hydraulic modulator on the EV) cost me $6 at the flea market, and it was still in the plastic wrap. Sorry, just trying to make you jealous.

If you can't find 10mm bubble flare plugs, you can make your own. Go to the auto dismantelers and cop four of the (good, clean, rust-free) ends off of a scrapped-out body. Cut the steel tubing about 1-2" from the flare nut, then crimp the cut end closed in a vise. Use a gas welder to weld up the crimp and you have a DIY plug. I've found them very handy when removing brake components to stop fluid loss and cap off orifices when cleaning in solvent baths. Never hurts to have a few around. I've also made a few caps from injector pressure lines to cap off injection pumps in storage and while cleaning (never cap off an injection pump and then try to rotate the shaft, you'll blow it apart!!!).

Good luck, there's nothing all that mysterious about hydraulic brakes, at least not until they stop working. Another reason my bus had air brakes!

[Golf Loon]

Gday Sharkey. The Electric Rabbit looks great. Would be an ideal delivery vehicle for a business. I reckon.
Wise words on the brake saga, I`m gonna make me some of those brake blankers!
The Bentley manuals are great, I have one for my Saab, so much more detailed than a haynes. Not available in Australia, we have to resort to ebay.