Dual 7" Servo

[peterux]

Originally Posted by oaktree11

Mike and Peter,

I am lying on my hotel bed having just arrived in Canada. I am away for 5 weeks so no worries with using my servo. I hope it helps.
John

Many thanks John!

[Grey V8 Pete]

Slightly off topic but, when I first apply the brakes after starting the engine there is a noticable “clonk” from the dual servo. This only happens the once and does not recur until after the engine is next started. All mounting bolts, clevis and pedal bolt have been checked and are secure. The servo operation remains the same and braking efficiency remains the same i.e. brilliant!

As a theory: The diaphragms seem to be on two in-line but physically separate push rods. I guessed this from the fact that before fitting the unit I could move the primary push rod sideways (when fitting the clevis over the fixed pedal bolt) which would not be possible if the rod was attached to both diaphragms. I wonder if there is more residual vacuum in one sevo chamber than the other so that as soon as the engine starts and supplies more vacuum, pressing the pedal makes one of the diaphragms respond a little faster than the other to take up the slack. Has anyone else noticed the same? Peter.

[Mike]

This may help us all to understand how a servo works. If anyone knows how to import the diagram direct into the form I'd appreciate the tip, rather than posting just the short cut.

http://auto.howstuffworks.com/auto-p...wer-brake2.htm

If you click on the pedal, it shows the valve at the rear opening to allow atmospheric pressure against the back of the diaphragm, whilsyt there is a vacuum on the front, creating the assitance to the pedal pressure.

As the pressure on the pedal is released, the valve at the rear closes and allows the vacuum to reform at the rear of the diaphragm, equalising the pressure on both faces, and allowing the pressure on the master cylinder to be released.

There should be a small amount of movement in the pedal from its rest position to allow this valve to open, before it meets with pressure from the diaphragm plunger and starts to apply pressure to the master cylinder.
The message appears to be do not try to take out the slack movement between the pedal at rest and the servo. ie do not set the brake switch too close to the pedal to prevent it coming back as far as it is intended.

Originally Posted by Grey V8 Pete

Slightly off topic but, when I first apply the brakes after starting the engine there is a noticable “clonk” from the dual servo. This only happens the once and does not recur until after the engine is next started. All mounting bolts, clevis and pedal bolt have been checked and are secure. The servo operation remains the same and braking efficiency remains the same i.e. brilliant!

As a theory: The diaphragms seem to be on two in-line but physically separate push rods. I guessed this from the fact that before fitting the unit I could move the primary push rod sideways (when fitting the clevis over the fixed pedal bolt) which would not be possible if the rod was attached to both diaphragms. I wonder if there is more residual vacuum in one sevo chamber than the other so that as soon as the engine starts and supplies more vacuum, pressing the pedal makes one of the diaphragms respond a little faster than the other to take up the slack. Has anyone else noticed the same? Peter.

[peterux]

I've been in my garage today, troubleshooting my brake problem with telephone support from Mike.Thanks for all the ideas!
First I removed the fluid reservoir cap - no change.
I removed the complete master cylinder - no change (so it's not the sleeve)
I then removed the clevis pin/bolt from the brake pedal and my stop light switch - no change
Then I removed the vacuum hose from the servo and the servo returned to its rest position. Interesting.....
Finally, I operated the servo completely removed from the car but still attached to the vacuum pipe. The problem is still apparent.
With Mike's help, I have emailed MBM describing the problem and we are awaiting their reply.

My next step will be to fit John's servo.

I'm sure Mike or I will update everyone when we hear back from MBM.

[AlanHogg]

Have been following this thread with interest and trying to find a decent schematic of a dual diaphragm servo. However if we just take the 'how stuff works' from Mike and knowing that the servo is not seemingly open to atmospere at any other source then it would appear that the air valve is not returning to it's rest position either by not enough free play in the brake pedal as has been suggested or that the air valve has a pin hole or some such allowing atmospheric pressure to act on the diaphragm. Are you sure that having the pedal connected to the servo actuating rod in the same manner as in the Marlin/Metro set up isn't preventing the air valve closing. My Cabrio was also assembled this way with the Metro servo but lined up perfectly with the new set up in the way Mike had intended.
I like others will be very interested in what MBM say

[Mike]

Peter/ Alan et al
MBM have responded very positively to Peter's video, and my request for a replacement.

Their response in full:

"Hello Mike,

I am sorry Mr. Edwards has had trouble with one of our boosters. When we sell product overseas, it can be difficult to warranty items because of the high cost associated with shipping them back. We need to be able to test the product and with the cost to ship parts back and forth, that just simply is not feasible. In most cases when we sell overseas, we do not extend a guarantee because of this. I appreciate the fact that he went to the trouble to make a video and show the issues that he is having and I can feel confident that what there are indeed issues with the booster, and with that, I have no problem sending you a replacement with your next order.

Sincerely,
Steve Brown
Sales Director"

I think that is as good and quick a response as we could have wished for.
It is a shame for Peter that he is the one to get a faulty servo. Sorry Peter!
But we will get it resolved to your complete satisfaction.

Mike

Originally Posted by AlanHogg

Have been following this thread with interest and trying to find a decent schematic of a dual diaphragm servo. However if we just take the 'how stuff works' from Mike and knowing that the servo is not seemingly open to atmospere at any other source then it would appear that the air valve is not returning to it's rest position either by not enough free play in the brake pedal as has been suggested or that the air valve has a pin hole or some such allowing atmospheric pressure to act on the diaphragm. Are you sure that having the pedal connected to the servo actuating rod in the same manner as in the Marlin/Metro set up isn't preventing the air valve closing. My Cabrio was also assembled this way with the Metro servo but lined up perfectly with the new set up in the way Mike had intended.
I like others will be very interested in what MBM say

[peterux]

Originally Posted by AlanHogg

Have been following this thread with interest and trying to find a decent schematic of a dual diaphragm servo.

Hi Alan,
probably a bit theoretical now but for anyone still interested, I did find this document on the web.
Page 47 onwards covers 'dual' or tandem diaphragm servos.
It may not be identical to MBM's design but I expect the principles of operation will be similar.

http://www.autoshop101.com/forms/brake05.pdf

...peter

[morris]

Originally Posted by Grey V8 Pete

Slightly off topic but, when I first apply the brakes after starting the engine there is a noticable “clonk” from the dual servo. This only happens the once and does not recur until after the engine is next started. All mounting bolts, clevis and pedal bolt have been checked and are secure....

I noticed that straight away but it happens everytime I depress the pedal. I've not tested properly yet as no running engine. it may go away like yours.

[oaktree11]

No problem from my side. Currently sitting at Vancouver Airport en route bloody Mongolia so I wont be needing it for a bit!
Thats the joy of the forum, we can all help one another - John

[denniswpearce]

Nice going Peter.

Good to have brakes you can rely on and not need acres of room between you and the car in front.

[cabrioman]

Peter

Did you end up using the reduction valve gismo for the rear drums or is that still out of the setup, intereseted as I have the same brake setup as you with discs at the front and drums at the rear.

John

[peterux]

Originally Posted by cabrioman

Peter

Did you end up using the reduction valve gismo for the rear drums or is that still out of the setup, intereseted as I have the same brake setup as you with discs at the front and drums at the rear.

John

Hi John,
I'm currently running with the standard BMW pressure reduction valve in the line to the rear brakes. This stops the backs locking up first, which I confirmed last week when testing my new servo.

The Residual Pressure Valve that I was trialling in the rear brake line is not currently installed. I removed it as part of my servo troubleshooting exercises. I am going to run the car in this config for a while before deciding to refit it.

I hope that helps?

...peter

[cabrioman]

Hi Peter

Thanks for the response. I am pretty certain my Hunter with its original Sierra/Metro servo setup doesn't have a limiter in the rear brake circuit. I guess I will have to have a play around with it when I come to fit the servo.

To be honest I would be amazed if there is a problem, if Mike and the others didn't have a problem with rear disc setups locking, rear drums should be fine as they are less efficient than discs.

The only problem might be extra pedal travel if there isn't a valve on the rear drums, the MB website seems to indicate this is the only reason they fit the residual valve. I will just have to make sure it doesnt cause binding when the brakes are hot if I decide to fit one.

John

[Grey V8 Pete]

The Sierra automatic brake adjusters, need the shoes to be fully "off" so the ratchet engages properly to adjust the mechanism to remove play as the shoes wear. An incorrectly adjusted handbrake cable (too tight) will cause this so I am sure that RPVs will do the same. I believe that you would only need to use RPVs if the master cylinder resevoir is below the brake cylinders as this could cause fluid to bleed back into the master cylinder. However I have a classic car (drum / drum) where the master cylinder is bolted to the chassis below the floor, so well below the wheel cylinders! There isn't a problem with bleed back because the master cylinder is fed from a remote reservoir up under the bonnet, so there is always a head of fluid above the brake cylinders. Peter.

[Mike]

Originally Posted by Grey V8 Pete

..............I believe that you would only need to use RPVs if the master cylinder resevoir is below the brake cylinders as this could cause fluid to bleed back into the master cylinder............. Peter.

Peter

This echoes what MBM say about the RPV.

Another point to make is that brakes require 900 to 1100psi to make drum / disc brakes work effectively. An RPV with only 2-3psi really should not cause any break issues. As you say its primary function is to prevent bleed back which effectively pulls the wheel cylinders away from the shoes by gravity, making the initial brake pedal travel greater.

Mike

[cabrioman]

Thanks for the input Peter.

I am not envisaging a problem, I will be setting things up without an RPV to start with, and only having a rethink if there is a problem.

John

[Sorton]

Mike, Peter

I'm afraid I have only just seen your PMs re the servo - a failure of my understanding of the site!

I will try and have a look at my servo this weekend and have sent you my contact number on a PM Peter as I would appreciate your input on how best to do it.

Sorton

[peterux]

Whilst browsing through my latest copy of Complete Kit Car I suddenly stumbled on some pictures I recognised !
Yep, it is an article written by Mike detailing the story of the dual diaphragm Servo trials and upgrades.

Well done Mike, good article!

[Mike]

Originally Posted by peterux

Whilst browsing through my latest copy of Complete Kit Car I suddenly stumbled on some pictures I recognised !
Yep, it is an article written by Mike detailing the story of the dual diaphragm Servo trials and upgrades.

Well done Mike, good article!

Hi Peter

I've not seen the article, but it was put together by Ian Stent, based on the articles which went into Pitstop. It will be interesting to see how much he has changed them?

Who's photos has he used?

[peterux]

Originally Posted by Mike

Hi Peter

I've not seen the article, but it was put together by Ian Stent, based on the articles which went into Pitstop. It will be interesting to see how much he has changed them?

Who's photos has he used?

Hi Mike,

it comes across as an article written by yourself, so i guess Ian has just used his editorial skills. I think it has three of your photo's and one on Ian's final installation.

It's good to get the 'dual booster' some wider publicity

...peter