410, 411, 412 servos

[dwomby]

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

[David C]

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.

[dwomby]

Originally Posted by David C

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.

Hmmmm. David, thank you. I have grave doubts about a 'balance bar' being fitted to my setup. The previous owner fitted a dual master and two servos but he went with the 1.9x boost servos. I doubt he knew about a balance bar and I don't recall seeing anything that would perform that function. Can you help me identify it? Where would it be? I can't see it mentioned in the spares book that I have downloaded.



David

[dwomby]

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

[David C]

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.

[dwomby]

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.

[Kevin H]

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