Brakes question

[gav]

Hi all

I have an unusual braking system but the question I have is generic. I wonder if anyone can assist please?

I have suffered this once before- when the brakes are warm, the brakes lock on and wont release. I recall that when it happened previously I found that the pedal set up (part of the unusual bit) wasn’t releasing so I adjusted it and the problem went away. My current check is to see whether this has repeated or whether the cause is something different. I have been playing with the system since I last encountered the problem (which was many years ago) and it is entirely possible that I have re set it to the day 1 settings rather than to the adjustments that I subsequently made.

I want to run a diagnostic and as part of that want to isolate the servo so my question is - if I temporarily remove the vacuum from the servo will that stop it working as a servo and act as a non servo installation (with a harder push required at the pedal to compensate)? I don’t want to dismantle components at this stage until I have worked through some diagnostics first.

As usual, all thoughts appreciated.

Thanks
Gavin

[Mazzini]

When the brakes are locked on, does a hard stamp on the pedal release them?

[rgh0]

Yes disconnecting the servo vacuum line makes it behave like an unboosted system with a heavier pedal and none of the vacuum / hydraulic control system functions so any potential lock on due to that cause should go away.

cheers
Rohan

[gav]

Thanks Rohan

As you say, this should help me isolate whether it is a problem with the servo.

All the best
Gavin

[MarkDa]

Brake servos can lock on like you describe apparently.
I'll have an internet rummage and find the article that described the problem and cure.

Edit I couldn't find the specific article I was looking for.
But there's a fair few threads on here.
If you have a Lockheed type servo it maybe the air valve sticking - add a small spring to cure.
If Girling the shuttle valve may not be as free moving as it should be - les is more in terms of lubrication apparently.

Hope this helps

[gav]

Thanks Mark

I am going to run the car as is but with the vacuum removed to isolate the servo. Assuming that this is the cause, I'll have a delve.

All the best
Gavin

[gav]

Sorry Mazzini - I missed your reply.

No a stamp on the pedal did nothing other than make me feel a little better

Thanks
Gavin

[Concrete-crusher]

I had a similar problem, also tried disconnecting the servo, and rebuilding the master cylinder

The eventual cure was a new master cyclinder

Steve

[gav]

My masters are pretty new so I disconnected the vacuum to the servo and with great trepidation went for a gentle drive.

My system has a bias pedal bar which I set to slightly front and checked that the pedal had sufficient play so that it didn't get jammed and tootled down the lanes. I have to say that it feels almost like waiting for a disaster and I was surprised when it didn't happen - albeit the brakes came back warm but not hot so it wasn't a full test..

Now that the system is working properly I really don't need the servo. The master cylinders are AP racing and there is no slop and very little movement needed to apply them (which I originally thought was the problem).

I need to go for a braver run when its quiet and see if the problem recurs.

I did wonder if it was the non return valve but I think that would cause the servo not to work as well (and it was certainly working well) - so in the Chapman spirit, if I do establish that it was the servo - it can go and I can save some weight and make some space in the engine bay.

Thanks for your help so far.

All the best
Gavin

[MarkDa]

You did say initially that your brakes were a bit different.
How does one servo boost twin masters?
Quite a lot of people manage without a servo - after all the cars aren't heavy.
Gain a bit more confidence and then as you say you can reduce the car's mass by a few kilos

[gav]

I was using a dual circuit servo like the one sold by car builder solutions.
I think it was originally for a lorry but surprisingly it isn't that heavy

The set up has a further tweak because my back brakes are Sierra and originally too powerful to the degree that I kept spinning at high speed events when I nailed the brakes. To solve this the rear circuit has an adjustable proportion valve which I set up after the last MOT thanks to a friendly MOT man.

The servo was one complication too many so I will be glad to see it go

[HCA]

Lockheed servos are well known for sticking on! Go into the Scimitar forum and there are pages on the subject... many there remove the lower seal of the balance valve, but I think this is wrong. I think you just have to keep the filter and valve clean.

[alan.barker]

They need to be positioned at certain angles
Alan

[MarkDa]

Apparently even at the correct angle the air inlet valve sometimes benefits from an an additional light spring.

[gav]

Would a sticking air inlet valve cause the brakes to lock on?

Thanks
Gavin