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| 8 & 10 cyl Bristol cars Type 407 onwards - restoration, repair, maintenance etc |
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23-04-21, 09:16 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, 11:53 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, 02:27 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, 02:11 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, 10:42 PM
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Quote:
At Bristol cars the servos are the same but we used to modify the master cylinder and if you didn't do the modification the rear brakes would stick on
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26-04-21, 01:28 PM
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Quote:
What was the modification? And was this on the 410?
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26-04-21, 02:49 PM
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Quote:
Do you mean the 'balancing bar' or 'compensating lever' fitted to the pedal on the 410 which used two master cylinders? Or was some other change made to a single dual master cylinder fitted on later models? If the latter, is that modified dual master cylinder available? It seems I have two choices here: leave the rear servo at 1.9x boost and fit a 4.25x boost at the front; or, replace both servos with 4.25x boost units but put a pressure limiting valve into the line from the servo to the rear brakes. The former is obviously a lot less work. David
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16-05-21, 06:46 PM
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I fitted a 4.25x boost brake servo (Lockheed LR18230) to the front brakes and left the 1.9x servo on the rear brakes. After days of fiddling to stop a fluid leak at the caliper, I finally put it back on the road today and took a VERY slow drive down the street to test the brakes still worked at all! It seemed ok so I took it for 30 minutes on the back roads.
The new servo for the front brakes gives over twice the boost the old one did and it really makes a huge difference. The pedal effort to stop the car is now similar to our modern Mazda and the stopping power is impressive for a car of this vintage and weight.. I'm really glad I made that change. David
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