410, 411, 412 servos

[David C]

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.

[Classic Bristol Car Parts Company]

Originally Posted by dwomby

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

David

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

[Kevin H]

Originally Posted by Classic Bristol Car Parts Company

David

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

What was the modification? And was this on the 410?

[dwomby]

Originally Posted by Classic Bristol Car Parts Company

David

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

Thank you.

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

[dwomby]

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

[Geoff Dowdle]

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

[dwomby]

Originally Posted by Geoff Dowdle

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

Thank you for that, Geoff. I did try that yesterday on dry, straight road. I am pretty sure all 4 locked up. The car stayed pretty much straight throughout but one pair left darker rubber lines than the others and appear to have started further back along the road.

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

[Geoff Dowdle]

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

[Kevin H]

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.