Dual to Single Circuit Braking System

[Rek]

A technical enquiry to see whether my logic is correct. RHD cars have two remote servo's with the same power ratio. In changing to a single circuit, would one servo work just as well as the pressure being applied when the pedal is pressed would be the same? Does the fact that a single servo is pushing on more fluid than each of two servo's reduce the effectiveness and therefore the effectiveness of the brakes.

Removing one of the cumbersome servo's would release space in the engine bay and make the plumbing a lot simpler.

Any advice gratefully received.

 

[nosmonkey]

I'm going the same route also, ditching both the standard servos and going for an off the shelf remote servo. Not sure what the ratio is on the standard ones but a larger single would do the job, my plan was a similar size to the originals albeit just the one and have a firmer brake.

With the larger braking set up and master cylinder to cope with them its having anyway it will help a bit with having more braking force to counter act the smaller vacuum assist. I'm not a fan of over assisted braking and steering as it is

 

[Rek]

Sounds like the right way to go. Have you found the right servo?

 

[nosmonkey]

Compact Dual Circuit Remote Brake Servo

2:1 power boost ratio @ 600mm Hg vacuum.

www.carbuilder.com

Guy, this is the one I was looking at

 

[Ohmess]

I'm reminded of the time when I wore the brake shoes on my Plymouth down so far that the piston on one of the rear brakes extended beyond its cylinder, allowing all of my brake fluid to escape onto the road. It gave me an appreciation for dual circuit brake systems I have not lost.

Make sure your emergency brake is in good working order.

 

[Honolulu]

Here in 'Murica we have little to no familiarity with dual vacuum servos.

If dual servos push twice as much as one by operating in parallel, you'll need to press the brake pedal twice as far to get braking. The geometry of your brake pedal, or length of stroke in the master cylinder, may not allow that.

If dual servos double the pressure by operating in sequence, MAYBE you can get by with only one of them, but I won't be along for the test ride.

I lost brakes once by effing up rear slave cylinder reassembly. Once the fluid was lost I had no brakes except for a handbrake getting stretched while trying to gain friction on a drum lubricated by Castrol GT4. Now THAT was a careful drive home, I tell ya. Sorta kind just as much fun as driving my old Bud without a clutch cable, or my bike when the clutch cable snapped. Is there a pattern here?

 

[Rek]

I currently have 2 X 2:1 remote servo's which double the pressure from the pedal (?) I suppose. Each pushes say half of the fluid in the system. If I replace these with one 2:1 servo, this will have to push twice the fluid as before, but will get twice the vacuum assistance as before.
Sounds like the answer is in complex fluid dynamics or some other such wizardry. I might just try it and see.

 

[Michael Kaye]

A technical enquiry to see whether my logic is correct. RHD cars have two remote servo's with the same power ratio. In changing to a single circuit, would one servo work just as well as the pressure being applied when the pedal is pressed would be the same? Does the fact that a single servo is pushing on more fluid than each of two servo's reduce the effectiveness and therefore the effectiveness of the brakes.

Removing one of the cumbersome servo's would release space in the engine bay and make the plumbing a lot simpler.

Any advice gratefully received.

I can't answer the technical points of your question but as you know I had my servos rebuilt recently. What Bill discovered was that the vacuum pipe on one was connected the wrong way round which essentially meant I was only using one servo.

I had just assumed that because it was an old, the braking was meant to be poor; not terrible but wasn't great and I had to think about my braking in advance! Maybe you could try something similar to see the effect but I'm obviously not recommending it

 

[BarnE9]

Compact Dual Circuit Remote Brake Servo

2:1 power boost ratio @ 600mm Hg vacuum.

www.carbuilder.com

Guy, this is the one I was looking at

Im on the verge of sending both servos off for a costly rebuild so this is interesting. This alternative seems to maintain the dual circuit system but simplify the plumbing - correct?

 

[nosmonkey]

Im on the verge of sending both servos off for a costly rebuild so this is interesting. This alternative seems to maintain the dual circuit system but simplify the plumbing - correct?

Looks that way, less complicated and less weight (those servos are not light!). I'm still welding sills onto mine so a long way off, not done much research into it just yet so I'm unsure how under/over assisted it would cause the brakes to be, but I figure one of these with an inline proportioning valve to the rear brakes off the rear circuit which then tees off to the calipers.
Also thought having the dual circuit set up diagonally (front left and rear right, front right and rear left), with 2 proportion valves for the rears. 2nd set up would at least allow for some more safety in the event of one circuit failing, but set up of equal braking pressure on the rear wheels would be a bit trickier (pressure gauges could be used for this or perhaps an MOT brake testing bay).
At least both options use more readily available components and cheaper than having old cores rebuilt, could still retain the standard twin pipe calipers up front with a T piece but I was going for single line Wilwoods.

 

[Drew20]

Ive always like the braking on early RHD BMs, there is a slight pause, but the braking feel is confident with the 4 pot fronts.
There are 6 pairs of pistons on the car, 4 pairs on the front and two pairs on the rear, so any system would need to supply fluid in multiple brake lines, 2 lines to each front caliper and 1 line to each rear.
Obviously T pieces can split the lines, but any new system would need to get pressure right at each caliper, and would need to have some redundancy if a brake line fails, or a m/c chamber loses pressure.
Is there mileage in mimicking how the LHD system works, with its single twin chamber m/c?

 

[Drew20]

Before ABS systems took over the role of proportioning valves, all cars with rear discs and diagonally split systems must have had either two propotioning valves, or a twin one. Trying to recall what my old mk2golf gti had, it only had one proportioning valve (I remember freeing it off to pass an MOT). So I assume it must have been a twin valve, as there were seperate lines to each rear caliper

 

[day66]

I’ve rebuilt the two servos on mine, though it will be a little while before the rest of the car is in one piece and I can see how well they work! I would think that when in good order the system should be fine, it’s an odd layout but FWIW a very similar set up was used on the DB5/6/S for example.

Certainly the cost of replacing a couple of servos is high and that‘s what led me to investigate a DIY approach. I’ve had both of mine stainless sleeved and sourced all the seals I needed, and that cost about £180 for the pair all in.

 

[E9madada]

I’ve rebuilt the two servos on mine, though it will be a little while before the rest of the car is in one piece and I can see how well they work! I would think that when in good order the system should be fine, it’s an odd layout but FWIW a very similar set up was used on the DB5/6/S for example.

Certainly the cost of replacing a couple of servos is high and that‘s what led me to investigate a DIY approach. I’ve had both of mine stainless sleeved and sourced all the seals I needed, and that cost about £180 for the pair all in.

Where did you get sleeved?

 

[day66]

The resleeving was done by Past Parts - about £45 each plus VAT and carriage

 

[Michael Kaye]

I have to agree with @day66 the diy approach does work and is a hell of a lot cheaper if you find the seals. I sourced mine from John "Yoda" Hudson ([email protected]). If you reach out to him he might even have a rebuilt pair for exchange.

 

[BarnE9]

The resleeving was done by Past Parts - about £45 each plus VAT and carriage

Thanks @day66 - for the less technically minded what exactly is stainless steel sleeving - presume an anti-corrosion measure? Also thanks Michael for Johns email - Ill see what he has available.

 

[day66]

The main body of the servo is, I think, made of cast steel, so unless you’re very lucky this tends to suffer from corrosion caused by moisture in the fluid. To reclaim the cylinder, it is bored out and then a tubular sleeve of stainless steel, about 1mm wall thickness, is pressed back in. This is honed back to size, so as well as reclaiming the body, it should not corrode again.