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  1. #1
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    770

    Give me a brake!

    Hi all
    I’ve been meaning to post something on this for some time, hopefully may help some others.

    I must admit one of the areas I've never been 100% happy with on my car is the brakes.
    I've had several incarnations of braking systems culminating in outboard rears and 4 pot calipers all round but still need quite a bit of pedal effort and particularly with today's busy roads this was just not filling me with confidence.

    Spared on my another post on here (http://www.cobraclub.com/forum/showthread.php?t=63162) I decided to look into fitting a servo.
    As discussed in that post Alan M fitted a Rover 25 servo but we discussed this and it had involved some pedal alterations to offset the clutch master to allow the pushrod to clear the servo body. As I didn't have access to welding equipment at the time I decided to look at other options, in the end I investigated the Mazda MX-5 servos (also mentioned in the post above).

    I found one on eBay which looked like it may do the job and was cheap enough to get as a bit of a "flyer" - Worst case scenario it was £10.40 I'd never see again!
    Turned out the servo was actually in quite good condition (although I did find another slightly later which was even better). It is from a MK 1 (or possibly 2) labelled up 833-04909, best I can tell is this is an 8" servo (outer diameter is 223mm max') with single diaphragm.
    MX-5's can be a bit of a minefield in the servo area at this age as there are many different versions, later ones (MK3) are more stable but the diameter is larger and would foul the clutch master and/or pushrod.
    I worked on the theory that the early MX-5s are similar weight to the Gardner Douglas so should provide adequate performance, in any case I’m not after massive assistance, just better than I have at the moment


    I took the apart the existing setup and extracted the pedal-box and clutch master.
    Master Cylinders-1-Reduced.jpg


    After careful measurement (several times) I made up a plate to fit onto the front of the bulkhead. I sandwich the bulkhead between this plate and the pedal box. Possibly not really required but I think it will help spread any load and it was also useful to use as a drill jig for the holes needed in the BH.
    I used this to drill out the hole pattern on the bulkhead
    Mounting plate on bulkhead.jpg

    and then used this to transfer the hole pattern to the pedal box (no picture unfortunately)

    With this done and the pedal box back out of the car I mocked up the assembly.
    servo-pedal box mock up-5.jpg

    servo-pedal box mock up-6.jpg


    The main problem here is that the Jaguar master fits to the BH with 2 holes arranged horizontally on centreline so they are reasonably "get-at-able" although the RH one does require the side of the pedal box to be spot-faced.
    The new holes are in a rectangular pattern with the upper ones sitting 30mm above centre, this makes it very difficult to get a wrench to them, particularly the RH one. For this reason I dropped the centreline 5mm.
    servo-pedal box mock up-2.jpg










  2. #2
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    770
    This is still tight but worked out just perfect.
    Top nut limited clearance 2.jpg


    With this all working on the bench (OK the floor) it was time to check how it fitted in the car. Would my careful measurements work out OK?

    Bolted the unit into the "cubby hole" and ran a straightedge across the front where the panel sits.
    Just perfect again!
    Front clearance check.jpg


    Assembling the kit inside the “cubby hole” was a bit of a chore. It’s all well and good everything fitting in there but you have to be able to twirl the wrenches to tighten the bolts. The sequence took a bit of working out and involved parting the brake master from the servo but all good in the end. It would have been a little easier except my arrangement feeds the rear brake line through the BH to a pressure reducing valve than back again. In hind sight I may have been better housing that inside the cubby hole but not a problem.

  3. #3
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    770
    Unfortunately it looks like I can't add any more photo's but I'll see if I can find a way around this and add them later.


    Was it worth the bother?
    Well this was actually quite an easy job and didn’t require the car to be off the road for a prolonged period. I did quite a bit of preparation prior to un-bolting parts from the car and it all went together as planned thereafter.
    In any case I’d say this is one of the best modifications I’ve made in the life of the car. The brakes give so much more confidence without being massively assisted.

    The assistance seems to drop off as the braking becomes more intense, I’m assuming this is due to the servo reaching the limit of its power (I’m assuming a given diaphragm diameter has a limit to how much force it can apply) but this is actually a good thing it means the brakes have plenty of initial power, much more than before but are very controllable as this is increased.

    For anyone with similar brake concernes I'd recommend it.

    Any questions please don't hessitate to ask.

  4. #4
    Join Date
    Sep 2020
    Location
    Wilts
    Posts
    19
    Thanks GDCobra. I am new to Cobra ownership (20 year old GD Mk3) and have the same feeling about brake performance. I haven't looked into the cubby hole yet but your report and photos give me confidence to have a go. I have bremsport callipers and big ventilated discs so it was disappointing to have so little bite from the brakes. It could be that the pads are too track oriented so I will try something softer as a first step. Any recommendations?

  5. #5
    Join Date
    Jul 2012
    Location
    NW England
    Posts
    770
    Quote Originally Posted by JMcL View Post
    Thanks GDCobra. I am new to Cobra ownership (20 year old GD Mk3) and have the same feeling about brake performance. I haven't looked into the cubby hole yet but your report and photos give me confidence to have a go. I have bremsport callipers and big ventilated discs so it was disappointing to have so little bite from the brakes. It could be that the pads are too track oriented so I will try something softer as a first step. Any recommendations?

    Thanks for the feedback JMcL.

    I'm not familiar with the Bremsport calipers (what size discs do you have?) but although big discs do give a slight improvement in leverage I'm not sure it would be night and day, there's also some argument to say that they may be a backward move as they won't stay warm as well. I did look at some different pads but all seemed to offer similar levels of friciton performance apart from "track only" ones.

    I initially tried some aluminium Brembo calipers on the front of mine, these were possibly better (and a benefit in unsprung weight) but all it did was make the fronts a little easier to lock up.
    This made me realise the the only way I could get more braking effect was to improve the rears so I went outboard with these and fitted the 4 pots, this did give me a better overall braking effect (I could pull more G under braking) but still required a lot of foot pressure, probbably OK in a track situation but and the road I want something a bit more "immediate" at times.
    With the new setup I seem to be able to 'plant' the front end into the road better under braking, don't know why, maybe the earlier build up of pressure helps the weight transfer, and I think I may have to reduce the rear presure now but that's not a problem as I have an adjustable reducing valve in the rear line.

    I know Andy at GD prefers un-assisted brakes but I think that depends on the requirement, for a road car I personally think this is better.
    I recently watched the assembly videos again which were from the very early days, obviously before I built mine (1997) and this does actually show a servo being fitted. It doesn't say what type and it does also mention that the cubby hole closure panel needed modifying with a "blister" to clear the end of the master cylinder. I guess I was lucky with my choice, it had minimal clearance at the front although I could have mounted it up to about 12mm further back but this would have probably meant modifying the pedal as the pickup point would need to move back or the pedal arm changing, or my legs shortening!

    If you do decide to look into if further, let me know if you need any help. I have a few more (larger) photo's and other details which you are more than welcome to.

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