Brakes & Servo

[SimonBanks]

I have a Delphi remote vacume servo unit fittef to my morris travellor by mr Morris Colchester when he was alive. The problem is as you run the engine it appears to apply the brakes on its own. any suggestions as to why this might happen.
Thanks
Simon

[MarkyB]

Maybe the top hat seal hasn't been removed from the master cylinder?
Or there is no play on the brake pedal.
Are they dragging or coming on quite hard?

Someone will be along shortly to say servos are the spawn of the devil, just ignore him

[bmcecosse]

That's me! Get rid of it - it's faulty..... Unless you have a leg strength problem - there's NO need for a servo in a small/light car like a Minor....... These cars were made for little-old-ladies to drive ! Top-hat seal is a disc brake nightmare - nothing to do with servo.

[MarkyB]

These cars were made for little-old-ladies to drive !

At 25 mph!
Anyone who drives the car after something modern will rate it as a death trap, the actual limiting factor in stopping is the width of the tyres not the amount of effort the pedal needs.

[bmcecosse]

Exactly - tyres - not servo - are what count.

[IslipMinor]

Simon,

First question is whether you have disc brakes?

If yes, then potentially one of two, or both problems, ignoring the servo itself:

Has the 'residual pressure' or 'top hat' seal been removed from the master cylinder?

Does the brake pedal have free play before moving the master cylinder rod itself?

If no discs, i.e. on drums, then only the second question is relevant.

[SimonBanks]

Drum brakes.
Not sure about the top hat.
There is only a little movement in the brake ped.

[bmcecosse]

Forget the top hat - it only applies with discs. You say the brakes are fine until you start the engine - ergo the fault lies in the servo. Clamp the vacuum pipe - do the brakes work ok now ?

[MarkyB]

There is only a little movement in the brake pedal

It needs at least half an inch, you may need to take the cover off the master cylinder and adjust the push rod.

[bmcecosse]

Indeed - but why only when the engine is started does it go wrong.......

[MarkyB]

Because the pedal is, in effect, being pressed?

[bmcecosse]

Not really - if it has 'some' clearance.

[kennatt]

are you saying that ,if for example,you jacked up the front then the wheels spin ok. then whan you start the engine the wheels now lock up without anyone touching the pedal. If thats the case than without doubt its the servo.Or does the brake stay on when you are actually driving the car,after using the brake.

[MarkyB]

Movement isn't the same as clearance.

[bmcecosse]

As mentioned above - I can only go on what we have been told - 'when the engine is started'.

[SimonBanks]

I have had this reply from the Servo manufactures;
"Hello Simon,

The incorrect operation of your remote servo, applying the brakes without the brake pedal being applied, is consistent with the reaction piston sticking in the servo assembly. The reaction piston is not returning fully when the pressure in the servo has decayed and this is holding the air valve open. When the vehicle is started vacuum is applied to the servo through the none-return valve on the slave cylinder side of the can, this vacuum should then be transferred via the bypass hole and air valve assembly to the top-side of the can, so the servo is in balance, but because the air valve is being held open, air pressure and not vacuum on the top of the diaphragm is pushing the pressure piston down the bore, so applying the brakes.

Hope the above is helpful

Best regards
Ian B Clarke
Delphi Lockheed Automotive DPSS"


I am therefore removing th Servo and going back to the standard brakes.

Simon

[bmcecosse]

Good plan! Servo should not be required in a small car like the Minor, unless the driver has leg problems of course.

[lowride stepside]

I love my servo assisted discs on my truck and there is no way i'de go back , especialy when its loaded at 80 mph .
Good ballanced standard brakes work well, but my ones definately work better , just clamp the vacume pipe in the snow

[bmcecosse]

If you have discs then yes you may need a servo - but unless you fit a pressure limiting valve in the rear line there is then a risk of rear wheel locking - which is really not a good thing.......

[lowride stepside]

A brake bias , I don't know if do have one but it stops perfect .you can drive along at speed ,release your grip on the steering wheel, apply the peddle and it just stops , perfect !