10 replies to this topic

[Geronimo]

I have run into a bit of an odd issue.  Started out with brakes staying on and the wheels never truly spinning freely.  The problem seems to of been building up over time.  I looked into rubber hose issues and so on, but what appears to be the issue is that the push rod on the brake servo is sticking out too far and engaging the master cylinder.  Previously I've never had them apart, but as soon as I did the wheels started spinning more freely.  There is little information on the Mini servo and I am uncertain if the push rod should extend beyond the opening of the servo or not and if there should be some pressure when you put them together.  Attached are two pictures.  One showing the push rod I am referencing and the second showing the gap if I was to put them together without pushing the piston in the master cylinder.

 

Any ideas are appreciated.

 

Thanks,

Jeremy

Attached Files

•  Servo push rod.png   437.26K   15 downloads
•  Servo - maste cylinder gap.jpg   50.8K   14 downloads

[panky]

Try dis-connecting the vac hose to the manifold and block the manifold connection off temporarily. If the brakes release then it's probable a leak passed the air valve.

Generic diagram of a servo operation

 

[Tobyrm1]

I had this with a new servo a few months ago. The brakes would never truly go free even with no vacuum connection.

Another new servo and the problem is sorted!

[Geronimo]

@Tobyrm1 - do you by chance know what the difference was between the two servos?

[Geronimo]

@panky - it isn't the vacuum.  If I take the servo complete out the push rod doesn't go any further back.

 

I was hoping someone would have an alternate servo and can confirm if their push rod extends beyond the opening of the servo.  It may be a master cylinder issue or something further down the line, but I am not sure what that would be.

[Tobyrm1]

@Tobyrm1 - do you by chance know what the difference was between the two servos?

No idea, sent it back to minispares as it was still in warranty and got a new one from somerford. It seemed to be fine if I disconnected the master cylinder off and then reconnected but when the brake was applied it never returned properly. Sorry I can't be of more help, from what I remember the rod coming out of your servo is sticking out more than mine did.

[tiger99]

Check that the pedal is not doing to the servo what the servo is doing to the master cylinder. The pedal return spring is vital and can possibly break. Never seen that, but it is a theoretical possibility. But also there is a known tolerance buildup problem. A plain master cylinder sometimes has to be shimmed to resolve the problem. You would need to shim the servo on its mounting fact to achieve the same.

The check is fairly simple, albeit in a thoroughly awkward place. All you need to do is check that the pedal, when released, is pulling on the push rod only, and not being blocked by any travel stop or the brake light switch. Indeed an overadjusted braje light switch is a common cause of problems like this on servo and non-servo systems.

Please do check the pedal before trying a new servo, as although it is not guaranteed to be the problem, there is no point on trying the expensive thing first. You may be lucky.

[Geronimo]

@tiger99 - thanks, I did check the pedal and it snaps back quickly when disconnected from the servo.  Springs all look to be good.  Even if I take the servo complete out of the car the push rod does not retract any further into the servo.

[tiger99]

Well maybe it is a tolerance problem between servo and master cylinder. Some cars have an adjustment facility on the push rod, probably not the Mini. You could try a shim between the two.

[Geronimo]

Thanks, I will either shim it or cut the rod down.

[tiger99]

Definitely worth a try. If that fails, a reconditioned servo may be needed but you have nothing to lose by trying, carefully. It should need only a very small amount of material to be removed.