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| 8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc |
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08-04-21, 08:46 PM
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410, 411, 412 servos
Does anyone have a reference for a suitable replacement for the servos on 410 to 412?
My Lockheed reference shows LR18503 in common with later Aston Martin V8 and Daimler Majestic Major. I cant find an updated model. I fitted an alternative suggested by a specialist back in the '80s but was disappointed that pedal pressure was much higher than original. Would really like to get back to something nearer standard. 
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11-04-21, 01:28 PM
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The previous owner of my 411 S5 installed two new Lockheed servos on my car. I have no comparison, but for me the car brakes well, with the right “feel”.
Unfortunately I can not see any markings on the servos telling me what type it is. Regards Thomas Willig
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11-04-21, 03:24 PM
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The previous owner of my 408 also installed two servos. For what it's worth, they were from Moss Motors part no. 117-312. They are by Powertune whose part number is RLE72696. I find them adequate but I'd prefer a bit more boost.
David
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11-04-21, 05:31 PM
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Those moss units have a 1.9 : 1 boost ratio. I think the originals on the 410 were LR18503 with boost of 4.5 : 1
I can't find replacements at 4.5 : 1 but have found 4.25, so that is looking like the best option at the moment.
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11-04-21, 05:34 PM
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Quote:
David
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11-04-21, 09:54 PM
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18-04-21, 04:01 PM
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Servo
I need a couple of servos and am looking to obtain them from CBCPC Ltd, who have many items it stock - and are re-ordering parts from their old manufacturers and suppliers.
Now they are no longer beholden to the parent company (switching monies between subsidiary companies meant a major disruption of the supply chain) parts that have not been around for five+ years should become available once more.
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22-04-21, 04:27 PM
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I think I will replace my 1.9:1 servos with the 4.25:1 servo but it has been suggested to me that I really only need to replace the front one. The front brakes perform most of the braking and a system of 4.25 on the front an 1.9 on the rear is what the Aston Martins of the era had.
Any thoughts on 4.25:1 on front plus 1.9:1 on the back being adequate versus 4.25:1 all round? The cost of a second 4.25 unit is not a factor really but the installation pain I anticipate is! David
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23-04-21, 07:09 PM
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The 410 has a balance bar to mechanically adjust the proportion of effort applied to each master cylinder. I assume this will be the case with your system.
If you have a 4.25:1 servo for the rear brakes it will need less pressure, so more can be applied to the front. As the manufacturer fitted with 2 x 4.5:1 and that gave a very good feel, I will replace with as near to original as I can. Once you have one servo removed it shouldn't be too much of a chore to do the second at the same time.
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23-04-21, 07:14 PM
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Quote:
David
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23-04-21, 07:52 PM
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I found parts on the 410 brake pedal page of the spare book called 'compensating lever'. Is that it? It looks like it sits inside the cradle or mounting turret that the brake pedal is hung from. My car definitely won't have that. It has a single inline dual master cylinder pushed by the brake pedal. Each line then pushes one servo. I cannot see any mechanism or valving that could be to proportion more force to the front brakes.
David
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23-04-21, 08:16 PM
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I was indeed referring to the compensating lever.
Best guess would suggest your first instinct is good for your particular car. I suggest you fit the 4.25:1 servo to the front and then get your brakes tested at an MOT garage. If the rears seem to be under performing relative to the front you can then take a good guess at how much extra boost is needed. Definitely better not to overdo assistance to the rear brakes. I have a 403 with discs on the front and many years ago I made the mistake of replacing the rear wheel cylinders with standard 403 items. I had to brake suddenly when I met another vehicle on a narrow lane and the rear of the car tried very hard to overtake the front. Very unnerving.
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23-04-21, 10:53 PM
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Thanks, David.
I have found another possible option. There are aftermarket, adjustable brake proportioning valves that I might be able to use. I will look into them some more over the weekend. Here's some info. https://www.speedwaymotors.com/the-t...ve-block/29302 As to an MOT, I live in Florida and this state, incredibly, has no mandatory emissions or safety checks! David.
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24-04-21, 01:27 AM
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The 411 was fitted originally with a dual master cylinder, whereas the 410 was fitted with two separate master cylinders - see here http://www.bristolcars.info/forums/8....html#post9519
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25-04-21, 01:11 PM
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and the saga drags on................................
I have now gone through a copy of the AM DB5 and DB6 parts manuals (I have the manuals, now all I need is one of their cars!). As far as I can see, they had NO mechanism to reduce the force on the rear brakes. They use a dual master cylinder with one line to a servo for the front brakes and one to a servo for the rear. Same setup as my car now has. There is no 'balancing bar', 'compensating lever', 'brake pressure limiter' or 'proportioning valve' visible or mentioned in the parts lists. So I thought they must have been relying purely on the lower servo boost to the rear brakes to reduce the chance of rear brake lockup in an emergency stop and I would do the same. Then I noticed they use the same part number for the front servo and the rear servo!!!!! David
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25-04-21, 04:29 PM
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I will be sending some old units off to be refurbished and will be holding at least one pair in stock. These will be sold on a exchange basics if anyone needs these just let me know. We have purchased the stock from the liquidator's of Bristol Cars via the Bristol Owners Club so we have lots of parts in stock and we are also ordering new and replacement stock.
Happy Bristoling Regards Graeme Classic Bristol Car Parts Company
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