Brakes locked on

[022Dave]

Dear Chums,

I had a problem today with the brakes, which I first had about three years ago. The brakes locked on, and wouldn't release until I disconnected the servo vacuum pipe. The front brakes seemed particularly hot & smelly. I then blocked off the pipe with a large bolt, before driving very carefully home.

Last time it happened I replaced the servo with a reconditioned unit, and also the master cylinder (new, not reconditioned). All was well for a short while, and then it started again. Opinion at the time was the servo being at fault, so it went back to the supplier (Paddocks) for testing. They couldn't find anything wrong, so I then fitted an old second hand servo, and it's been fine ever since. I’ve had the connecting rod set to the right length when the car was at Faversham Classics when I had all new Wilwood calipers, discs, pipes etc. fitted some months ago. It has always had a good firm pedal, so nothing has changed which would point to the master cylinder being faulty, and as the servo was disconnected for the journey home without locking on again, it seems to point to the servo once more.

Is there a release valve in the servo that could be causing the problem, or is there something else I should be looking at?

Dave

[martin]

Hiya Dave, what you're describing, sounds exactly like the result when the brake hoses start to collapse, they close up internally and won't let the fluid return to the system. I note you say you've replaced them, but with what ? It doesn't sound like the servo, it may be that during the servo replacement process, the hoses relaxed and started working again, giving the impression it was the servo. It's a very simple device, the only valve I can think of is the non-return to the manifold, if that fails it won't work. Cheers, Martin.

[022Dave]

Thanks Martin. Faversham fitted brand new steel-braided flexible hoses on all four corners, so I doubt it is those, and both fronts were equally hot & smelly, which I wouldn't expect if it was a hose collapsing. The servo is old, hence my thinking it's that at fault.

[Ian Durrant]

Hi Dave,

Are you using silicon fluid by any chance?

Ian

[GDPR]

BMW servo works well......

[Andrew S]

Hi Dave. Could there be any way that the back brakes are being blocked off causing the front brakes to take the loads and then over heat? That could explain the heat and smell. Can the PDWA switch cut the fluid off to the bake brakes? Does your brake pressure differential light work to tell you of any variance?

[022Dave]

Hi Dave,

Are you using silicon fluid by any chance?

Ian

No Ian, just the regular stuff that eats paint work. It was all flushed and replaced with the new brakes.

[022Dave]

Hi Dave. Could there be any way that the back brakes are being blocked off causing the front brakes to take the loads and then over heat? That could explain the heat and smell. Can the PDWA switch cut the fluid off to the bake brakes? Does your brake pressure differential light work to tell you of any variance?

Hi Andrew, the rears were warm, and the hand brake works fine. The PDWA light didn't come on, so I'm guessing it's not that. I think I'll stick to cameras; Nikon's don't give problems!

[Ian Durrant]

No Ian, just the regular stuff that eats paint work. It was all flushed and replaced with the new brakes.

Ah OK. it's just I have some seriously second hand information (though from more than one source) that has had exactly that problem when switching to silicone. Clearly that's no help to you though.

[Andrew S]

I finally bought that lens last week and very pleased, thanks for your help and advice

The part that I don't understand is that the car pulls up evenly I presume, so it has to be a central controlling item such as master, servo or switch

[022Dave]

I finally bought that lens last week and very pleased, thanks for your help and advice

The part that I don't understand is that the car pulls up evenly I presume, so it has to be a central controlling item such as master, servo or switch

There was no uneveness; I braked onto a roundabout, then found I had to tread fairly heavily on the loud pedal to get it to pull away again. I also had to accelerate to go down a hill, which is when I pulled over to see what was wrong! As soon as the vacuum pipe was disconnected, the brakes released.

[GTJones]

Sounds as if the Vacuum valve is not operating correctly, when you release the brake pedal the valve should allow vacuum to the rear of the diaphragm thus releasing the pressure on the master cylinder

Check out this web site as it shows how it works

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm

Gary

[Staggard]

Dear Chums,

I had a problem today with the brakes, which I first had about three years ago. The brakes locked on, and wouldn't release until I disconnected the servo vacuum pipe. The front brakes seemed particularly hot & smelly. I then blocked off the pipe with a large bolt, before driving very carefully home.

Last time it happened I replaced the servo with a reconditioned unit, and also the master cylinder (new, not reconditioned). All was well for a short while, and then it started again. Opinion at the time was the servo being at fault, so it went back to the supplier (Paddocks) for testing. They couldn't find anything wrong, so I then fitted an old second hand servo, and it's been fine ever since. I’ve had the connecting rod set to the right length when the car was at Faversham Classics when I had all new Wilwood calipers, discs, pipes etc. fitted some months ago. It has always had a good firm pedal, so nothing has changed which would point to the master cylinder being faulty, and as the servo was disconnected for the journey home without locking on again, it seems to point to the servo once more.

Is there a release valve in the servo that could be causing the problem, or is there something else I should be looking at?

Dave

As you say it points to the servo holding on if you then disconnect the vacume hose it then releases what I don't understand is you say you " set the connecting rod to the right length " how did you do that? Graham

[022Dave]

Sounds as if the Vacuum valve is not operating correctly, when you release the brake pedal the valve should allow vacuum to the rear of the diaphragm thus releasing the pressure on the master cylinder

Check out this web site as it shows how it works

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm

Gary

Hi Garry, that's my thought, but can the valve be changed, or is it another 'new' servo? Dave

[022Dave]

As you say it points to the servo holding on if you then disconnect the vacume hose it then releases what I don't understand is you say you " set the connecting rod to the right length " how did you do that? Graham

Faversham set it up for me. Apparently the rod is adustable so that servo's and master cylinders can be set up to get rid of excess travel felt through the pedal, and give the correct piston operating travel in the master cylinder.