Opinions please Brake master cylinder and servo operation.

[john 4d]

With the engine turned off depressing my brake pedal seems to offer quite firm resistance, which I've taken as a well bled system, however. With the engine running the pedal goes soft and I'm able to depress the pedal quite a way down before resistance is met, pumping the pedal offers a degree of firmness but is accompanied by a wheezing, like air bellows venting. The brakes work but compared to my Passat (not fair I know) they seem feeble.
If my explanation makes any sense and rings any bells I'd appreciate some expert advice please.
John4D

[V Mad]

Your brakes may seem quite firm (engine off) but they may still have some air in the fluid. I think the action of the servo accentuates any sponginess.

If brakes are ineffective, check condition of discs, especially the inner surfaces that you cant easily see; corrosion can cut down the effective area. Also rear brakes are important too. Check if shoes are making contact over most of their area.

[73stagman]

I think the scenario is as follows

And I hope i'm not teaching the taught but for the sake of it here goes (A board and some chalk would be useful)

You are probably comparing four wheel disc brakes (Passat) to two discs and two drums.
The servo unit (its a vacuum assisted system ) allows much higher hydraulic pressure to be applied with the same pedal force to the brake operating cylinders (Wheel cylinders or caliper pistons) but only when the engine is running as the vacuum is produced by the flow of air through the inlet manifold (You may get a small amount of extra force applied if you apply the brakes two or three times after you switch off the engine whilst the vacuum in the servo is replaced with normal air pressure) In the older Stag brakes the extra movement in the pedal comes from the rear drums expanding and any slack (tiny it may be but the hydraulic system pressure will allow you to "feel" it) being taken up in the shoe mechanism. As the circular faces of the shoes are forced against the inside of the drum they will move and settle to the surface diameter of the drum until they cant go any further. This extra movement is also "felt" when the engine is started if you already have the brake pedal depressed. In fact it is how you know the servo is doing its job.
It does sound odd though that you can hear the servo working (wheezing?) maybe you do have an air leak in the servo system rather than trapped air in the brake fluid system.
The servo draws vacuum from the inlet manifold with a single flexible connection. It needs to be in good condition, not perished, not too soft, cracked or too hard. There is also a rubber seal around the fitting into the servo. It also needs to be in sound condition.
If the brake pedal goes hard immediately (on the very first pedal action once the engine has come to a stop) you stop the engine then I would suggest that the vacuum is not being held by the servo at all and would indicate a leakage. It could be the actual servo unit diaphragm (it wont be completely blown or you wouldn't notice the effect the servo has at all because it wouldn't be working)

[martin]

Hiya John, with the engine running, press and hold the brake pedal firmly, does it sink further than normal ? If yes, the master cylinder seals are giving up, if not, it's just a matter of bleeding out the air. The "wheezing" is just air passing through the servo filter inside the car. Martin.

[Duncan Skye]

Hi John,

I think Martin is right, but note also that if the seal in the master cylinder is letting by, the fluid is likely to collect within the Vacuum unit, on the Master Cylinder side of the diaphragm.

Are you having to top up the brake master cylinder with fluid? If not, the seal is probably not letting by and you're going to have to repair it. If you are having to top it up, it would be worthwhile seeing if you could (using a clear plastic tube) suck out any fluid that may be lying inside the bottom of the servo unit when you have the master cylinder out to repair the seal. If there is a lot of fluid inside the servo unit, this will have the effect of reducing the 'swept volume' of the unit, meaning the full effect of the vacuum cannot be applied and you will not get the full servo action that the designer intended. Bear in mind the age of our cars - a previous owner may also have had to repair a leaky seal and not checked for fluid collecting in the vacuum unit. I know when I did mine, a surprisingly large volume of fluid came out.

It may also be worthwhile checking inside the car as well. Run your fingers or a cloth under the rubber cap that surrounds the operating rod from the brake pedal. If its wet, then leaking fluid may have found its way past (or through) the diaphragm. Keep an eye on it and any seepage should disappear after fitting new Master Cylinder seals.

Hope that helps,
Duncan, Skye.

[v8wgm]

Hi John,

Yours scenario sounds exactly like mine.

You mention that the pedal drops quite a bit when the engine is started. If you press the pedal as hard as you can & then start the engine, if it drops a lot then I would say there is air in the system. I just measured my pedal drop & it is 4 to 5mm.

My vacuum system exhausts itself after 3 presses which I feel is normal.

One method I used to check the master cylinder was to disconnect the pipes from the m/c unit, fit bleed nipples & bleed. I found the pedal was like a brick wall & would not sink over the period of a couple of minutes. Then I got bored! Whilst continuing to hold, I started the engine & got no pedal drop whatsoever on start up & again I could hold the pedal with no drop at all over a period of time.

Gavin

[john 4d]

Any chance you lot could move down to Gloucester and give me a hand, a second opinion of the symtoms would be appreciated!
John4D

[john 4d]

I'm serious!
John4d

[martin]

I'm serious!
John4d

I quite like Gloucester, how you fixed for a house loan ? Martin.