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Is your master cylinder new or used? I am thinking that there is still residual hydraulic pressure in the lines? Is the return port blocked? Is the hydraulic fluid free to bleed back into the resevoir when you release the brake pedal? Try the brakes with the reservoir cap temporarily loosened. Just a thought. Peter.
Is your master cylinder new or used? I am thinking that there is still residual hydraulic pressure in the lines? Is the return port blocked? Is the hydraulic fluid free to bleed back into the resevoir when you release the brake pedal? Try the brakes with the reservoir cap temporarily loosened. Just a thought. Peter.
Peter,
thanks for the ideas...
I have a brand new m/c.
Is the return port blocked? The rear brakes are releasing fully when I pull the brake pedal back to it's stop position so I think the m/c is OK.
But I'll will try your idea of loosening the reservoir cap to see if it makes any difference.
Mike,
I'm thinking of operating the brake pedal and servo (with the engine running) but with the master cylinder removed. Do you think this will damage the servo?
I want to see if the servo returns under it's own spring without the help of the m/c. It will also eliminate any issue with my m/c pushrod.
The new servo is extremely sensitive - the smallest dab of the pedal operating the brakes. Not unusual in a modern vehicle. What if it is just the residual pressure applied by the unloaded pedal that is causing the servo to operate slightly? That combined with your drum brakes could be the problem. From this thread it is just the rear brakes that are binding - yes? That is the main difference between your installation and everyone else that has used it I believe. You have drums at the rear - everyone else has disks.
If it is just the residual weight of the pedal, then a return spring should resolve it.
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.
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.
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.
Grey V8 Pete wrote:
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.
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.
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
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
AlanHogg wrote:
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
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
Great result....eace:
Nobody's manufacturing processes can be 100% perfect so there's always one unlucky person that gets the faulty one. But great customer service from MBM (and of course, Mike!) in agreeing to an instant replacement.
You can't argue with that.
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.
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.
I'm now trying to get this feature into the next issue of the mag. I need all the pics that were used over the two issues [Pitstop -] please. Can you send them to me, either in three or four emails, or zipped into a folder and emailed to me via [url]www.largefilesasap.com[/url] This service is free and works well.
All the best
love the domed cap nuts you've used on the servo Jason, I've been looking for places to put some of those myself though I guess you need some wave/spring washers underneath to stop them working off?
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
A quick update on my brake servo issue. I collected a replacement servo from Mike at the Stoneleigh show. (Actually, it is John's servo and he will get the replacement servo from MBM with the next batch of orders).
I've now finished re-installing it and I can report that everything is working as expected with no issues.
I've only had time for a short test drive just to make sure every was OK after bleeding the brake system.
Result - High performance brakes (as expected!) and no issue with the rear drum brakes. After the run, I stopped my car facing up hill and I could tell the brakes were not binding as the car would immediately roll backwards when the brake pedal was released.
Thanks again to both Mike for sorting out a repalcement from MBM and to John for letting me have his unit. Thanks guys.
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
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.
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.
eace: 
Thanks guys.