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Back in the '80s I was told my old 4.5:1 units were beyond repair and I was sold a more modern pair of servos that must be a much lower ratio as the pedal pressure required for given braking effort was far higher straight after they were replaced.
The long and short is that I don't have suitable units to trade in but would like to get the brakes back to something approaching original spec. At the moment a pair of 4.25 : 1 units look like my best bet. Other suggestions or reasons not to do this gratefully received. |
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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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What was the modification? And was this on the 410? |
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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 |
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 |
David,
When you have confidence in the brakes I suggest you take the car out on a quiet country road at say 60 mph and when safe do an emergency stop . You might find the front brakes might lock up!!!!!!!!!!!! BCL would have done a lot of testing of Servos in conjunction with Lockheed for more than 10 years to decide on the original servos of the 410. Geoff |
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I do not know how to tell if one pair locked before the other. How do you know if front or rears locked first? I am guessing the darker line was the fronts as they carry a lot more weight in a hard stop and that line started further back (about 3 yards before the lighter lines appeared) but does that mean they locked first? Any advice, please? David |
David ,
I'm certainly not a brake expert/engineer but I would not want brakes on my Bristols to lock on a dry road . Imagine what would happen on a wet road if you had to brake suddenly while going through a corner . It could be a disaster . Years ago I had a 411S1 with standard brakes in good condition . They worked very well for normal road use . Pressing the pedal half way gave average to good braking, then pressing harder really pulled the car up , with no fuss or lockup. It gave good feel and was not over assisted. Drive carefully !!!!!!!!! Geoff |
I had a 3.0 Si BMW on which I could lock up the wheels on a dry road - it happened on the Finchley Rd in London and I skidded into the back of a Porsche!
I'm sure I've had other cars with standard brakes on which I could lock up the wheels on a dry road. |
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