Announcement

Collapse
No announcement yet.

Improving the braking on my Cabrio

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Improving the braking on my Cabrio

    The Marlin Cabrio V8 is a pretty fiery beast, particularly with a modified engine, so if, like me, you’ve been using it with the Cortina Mark IV servo and master cylinder, you will know that uneasy feeling that the brakes are not quite up to the job. I understood the Cortina gear was necessary because the larger engine took up more space under the bonnet, but you need better braking with the more powerful engine, not worse!

    When I saw on the Marlin Owners Forum that Mike Garner was offering a 7” dual servo, I lost no time in obtaining one. From the comments on the website, it seemed the best way to improve my braking, but I was hoping that it was not too complicated to install. In the event, it was surprisingly easy. Installing the servo on the bulkhead only required some easing of the holes I had drilled for the Cortina servo in the first place and the Cortina master cylinder (I had purchased a new one on the internet) required the same easing to fit neatly on the front of the servo. Mike included an adaptor plate with the servo, but I did not need this. As the new servo was only ¾” longer than the Cortina one, I was able to fit the existing pipe work. This was all accomplished quickly and easily and on my first test drive I was astonished by the improvement in the braking. I was able to lock the brakes with fierce braking, but with normal pressure, the braking was very powerful, smooth and well balanced. The only problem was that there was excessive brake movement to activate the brakes. The reason for this, I believe, was that the Cortina master cylinder had a bore of 18mm, compared with 22mm on the Sierra. This creates greater pressure, but takes longer pedal travel to achieve it.

    Mike also supplied a clevis with the servo, which would have been very useful to attach to the brake pedal via a threaded rod, as it would have made the pedal linkage adjustable, but I could not find a way to do this. I had a bolt welded to the side of my brake pedal, rather than a hole, so in the end, I adapted the Marlin block used in the original build, but shortened it in order to raise the pedal to the same level as the other instruments. This works perfectly well, but it is not adjustable.

    In order to reduce the pedal movement, I turned my attention to the front end of the servo, removed the existing pushrod and replaced it with a 40mm long 4mm dome-headed bolt. I adjusted it so that about 20mm was showing and replaced the master cylinder. I was afraid I had overdone it so that the brakes would not release when the pedal was eased, but, as luck would have it, it worked beautifully. My brakes work just as well as before, but with reduced pedal travel. I had planned to make this the first stage of a two stage plan; the second stage being converting the rear brakes to disc, but quite frankly, I think the installation of the new servo had done the job quite without further modification.

    I cannot comment about using the 7”dual servo with other components used in the braking, but for owners with the Cortina servo and master cylinder, it is very effective and is quite easy to install. I am extremely grateful to Mike Garner who gave me some invaluable “after sales” advice and for putting me in touch with other Marlin owners who had dealt with similar problems in their conversions.

    One last comment; I know some owners with the V8 engine has experienced some problems as the master cylinder (being a little further forward) fouls the air filters on the SUs.. I had no such problem because I have Holly carburetor which sits happily between the rocker boxes!

  • #2
    Re: Improving the braking on my Cabrio

    Fab! Great work - any chance of a piccy of the installation?
    Cheers

    Comment


    • #3
      Re: Improving the braking on my Cabrio

      Follow up to Mike Jeans Dual Servo Installation.

      Mike was very impressed with the improvement in the stopping ability generated by the Dual 7" servo, but later reported he had an issue with his brakes sticking on, and then over heating - this has also been experienced by at least one other Marlin owner when first installing the dual servo.

      Whilst on holiday down in the South West, I suggested to my better half that we could "just drop in whilst en route" to meet a fellow Marliner, and see if I could offer any help. Mike's wife Joyce provided excellent scones, jam & tea, whilst Mike & I worked on his servo set up.
      I am pleased to advise that Mike has now driven his Cabrio over 100 miles since the minor changes we made, and there appears to be no binding and he has brakes to match his horsepower!

      The changes we made were simple and small, but I would encourage all other Dual servo adopters to be aware.

      The Marlin pedal box does not align the brake pedal with the master cylinder (seen here with the original Metro servo)



      It is therefore important to allow the servo input rod to align with the servo and NOT with the pedal. This either requires the holes in the clevis to be elongated slightly, or better still fit a rose joint.

      As Mike had experienced problems we decided a rose joint would be a better engineered solution. This requires a 3/8" UNF thread, but a 10mm eye hole to fit the Marlin brake pedal bolt. Unfortunately all the female rose joints advertised are not quite long enough to be fitted straight on to the servo push rod. The simplest solution is to obtain a male threaded rose joint (3/8" UNF x 10mm eye) and a 3/8" UNF collar.

      IMG_4693.jpg

      These can then be screwed straight on to the servo

      IMG_4695.jpg

      An important part of the set up is to set the adjuster screw so that it is just short of touching the master cylider push rod. On all the Sierra master cylinders I have come across this appears to be flush with the mating surface of the servo (or adaptor plate if used). This should be checked either by measuring the depth of the master cylinder pushrod relative to its mating face, or by offering the servo to the master cylinder and extending the adjuster rod untilthe two surfaces touch with out starting to apply pressure to the master cylinder input shaft. This is easy to detect as the adjuster begins to be over extended the servo and master cylinder mating surfaces "rock" on the pivot of the push rod/adjuster. By backing it off at this point the optimium length can be determined accurately.

      IMG_4697.jpg

      Note - over extending the output adjuster means the master cylinder never returns to its rest position, so can not release the brake fluid back to the reservoir; this then leads to a build up in pressure in the brake system. In both cases where brake binding had been reported the adjuster screw was set too long.

      The likely reason for the owners over extending the adjuster screw is interesting.

      The new Dual servo generates more boost than the Metro servo, therefore the pedal travels further for the same foot effort, and feels like the pedal travel is longer. In both cases the adjuster screw had been deliberately extended by the owners to try to compensate for the longer pedal feel - with the unfortunate and unexpected consequence that the brakes began to bind.

      The updated Dual servo kit now looks like this:

      IMG_4698.jpg

      Any owner with a non Ford Sierra servo will be impressed by the improvement this brings to their brakes: it finally makes our cars safe to drive in modern traffic.

      I currently have two servo kits remaining, and do not intend to source any more as over 50 owners have now adopted this servo, and I am guessing there will not be sufficient new owners to make ordering a batch worthwhile.........so when these are gone, they are gone!

      Regards to all who helped with this project

      Mike
      Last edited by Mike; 10-08-13, 07:02 PM.

      Comment


      • #4
        Re: Improving the braking on my Cabrio

        [QUOTE=Mike;26032]Follow up to Mike Jeans Dual Servo Installation.

        Mike was very impressed with the improvement in the stopping ability generated by the Dual 7" servo, but later reported he had an issue with his brakes sticking on, and then over heating - this has also been experienced by at least one other Marlin owner when first installing the dual servo.

        Whilst on holiday down in the South West, I suggested to my better half that we could "just drop in whilst en route" to meet a fellow Marliner, and see if I could offer any help. Mike's wife Joyce provided excellent scones, jam & tea, whilst Mike & I worked on his servo set up.
        I am pleased to advise that Mike has now driven his Cabrio over 100 miles since the minor changes we made, and there appears to be no binding and he has brakes to match his horsepower!

        The changes we made were simple and small, but I would encourage all other Dual servo adopters to be aware.

        The Marlin pedal box does not align the brake pedal with the master cylinder (seen here with the original Metro servo)



        It is therefore important to allow the servo input rod to align with the servo and NOT with the pedal. This either requires the holes in the clevis to be elongated slightly, or better still fit a rose joint.

        As Mike had experienced problems we decided a rose joint would be a better engineered solution. This requires a 3/8" UNF thread, but a 10mm eye hole to fit the Marlin brake pedal bolt. Unfortunately all the female rose joints advertised are not quite long enough to be fitted straight on to the servo push rod. The simplest solution is to obtain a male threaded rose joint (3/8" UNF x 10mm eye) and a 3/8" UNF collar.

        IMG_4693.jpg

        These can then be screwed straight on to the servo

        IMG_4695.jpg

        An important part of the set up is to set the adjuster screw so that it is just short of touching the master cylider push rod. On all the Sierra master cylinders I have come across this appears to be flush with the mating surface of the servo (or adaptor plate if used). This should be checked either by measuring the depth of the master cylinder pushrod relative to its mating face, or by offering the servo to the master cylinder and extending the adjuster rod untilthe two surfaces touch with out starting to apply pressure to the master cylinder input shaft. This is easy to detect as the adjuster begins to be over extended the servo and master cylinder mating surfaces "rock" on the pivot of the push rod/adjuster. By backing it off at this point the optimium length can be determined accurately.

        IMG_4697.jpg

        Note - over extending the output adjuster means the master cylinder never returns to its rest position, so can not release the brake fluid back to the reservoir; this then leads to a build up in pressure in the brake system. In both cases where brake binding had been reported the adjuster screw was set too long.

        The likely reason for the owners over extending the adjuster screw is interesting.

        The new Dual servo generates more boost than the Metro servo, therefore the pedal travels further for the same foot effort, and feels like the pedal travel is longer. In both cases the adjuster screw had been deliberately extended by the owners to try to compensate for the longer pedal feel - with the unfortunate and unexpected consequence that the brakes began to bind.

        The updated Dual servo kit now looks like this:

        IMG_4698.jpg

        Any owner with a non Ford Sierra servo will be impressed by the improvement this brings to their brakes: it finally makes our cars safe to drive in modern traffic.

        I currently have two servo kits remaining, and do not intend to source any more as over 50 owners have now adopted this servo, and I am guessing there will not be sufficient new owners to make ordering a batch worthwhile.........so when these are gone, they are gone!

        Regards to all who helped with this project

        Mike

        Yes, My Cabrio's brakes are still extremely effective, but the mistake I made with the original installation was to fit a 9mm bolt to the front of the Dual Servo that compressed the master cylinder too early. I was aware that this might not allow the pressure to drop when the brake returned to it's "rest position", but at the time of the installation, I thought this might be immediately apparent. It seems as though the pressure builds over time, because the brake binding problem came later.

        As a result of fitting the original "plunger" I still have quite a bit of pedal travel, but I can live with this...for a while at least!

        Mikejj

        Comment

        Working...
        X