10 replies to this topic

[lildeucecoop72]

Hi I am uprating my brakes from single to dual line.

The idea is to make the car safer after I drove another car (not a mini) with brake faliure on a single line that was pretty scary but luckily ended OK.

 

I have:

 

• A dual MS  - yellow tag type (GMC227)
• A brake limiter (FAM7821)
• A remote servo
• Pair of Metro front calipers/with solid disks for front
• Pair standard drum brakes on rear.

 

I want to run the servo on the front system only, as this takes most of the braking force, and I've seen that lots of people have done this.

 

Below is how I saw the setup (Servo after the limiter) with these parts.

 

I couldnt find any info here on this, so my question is would this work OK or should I have the servo before the limiter? (Between the MS and the limiter).

 

Thanks again,

 

EDIT: nov 2022. This is not the system I ended up using, see further down in the post :)

 

 brake setup 2022.JPG   31.52K   2 downloads

Edited by lildeucecoop72, 16 November 2022 - 02:40 PM.

[Maccmike8]

Looks good to me.

[sonscar]

Why not fit the later dual master cylinder and servo combination,seems to Be the easiest route but I am open to sensible reasoning as to why not.Steve..

[lildeucecoop72]

Why not fit the later dual master cylinder and servo combination,seems to Be the easiest route but I am open to sensible reasoning as to why not.Steve..

Yes considered that, but would like to use the parts I have, and also as I understand it you need to changed the pedal box/ it won't fit the '72 mk3?

Edited by lildeucecoop72, 12 May 2022 - 01:59 PM.

[Ethel]

I think the remote servo was used like you intend, on the front of a dual circuit system on some overseas produced version, possibly Innocenti?

 

 

The specific location conundrum is better than sudoku...

 

The valve will limit the rear at the same pressure, whether it's made with the assistance of a servo, or not, so I don't think it'll  matter in normal use.

 

The safety element that'd make the FAM7821 worth keeping is if the front fails, the rear braking force won't be limited. So I reckon where the servo goes best will depend on what effect it could have on the line pressure through the FAM7821 if there was a failure.

 

The servo uses increased pedal pressure to lift a little plunger that opens the air valve to let atmospheric pressure behind the diaphragm. As the diaphragm drives the servo's fluid piston forward the fluid volume behind the piston increases and the valve plunger can drop, closing the air valve  - unless the pedal pushes more fluid out of master cylinder instead. So, as long as there's pedal pressure the servo will isolate the brake circuit between the brake master & the servo from the lower part of the line, between the servo and the calipers/wheel cylinders.

 

If the servo was downstream of the FAM7821, you could have a pressure failure after the servo while the servo maintains fluid pressure before it that operates the brake limiter to limit the rear braking effort . Not ideal.

 

If the servo was before the FAM7821, lost pressure after the limiter would still prevent the limiter from limiting the rear brake effort and the servo would prevent fluid loss as long as it is operating. Better than just the FAM7821 without the servo.

[lildeucecoop72]

 

If the servo was downstream of the FAM7821, you could have a pressure failure after the servo while the servo maintains fluid pressure before it that operates the brake limiter to limit the rear braking effort . Not ideal.

 

If the servo was before the FAM7821, lost pressure after the limiter would still prevent the limiter from limiting the rear brake effort and the servo would prevent fluid loss as long as it is operating. Better than just the FAM7821 without the servo.

 

Thanks Ethel, good reasoning there, not something I had thought of!

 

I agree that if there is a faliure down-stream of the servo placed after the limiter this would result in the limiter 'doing its job' regardless and limit the only working rear line. This is not ideal, particularly in an emergency situation. The likelyhood of faliure is around the 'moving' parts after wear and tear, or areas that can be subjected to potential trauma (hoses to calipers or caliper seals).

 

As part of this conumdrum I have another question regarding the other option which must be considered and my reasoning for the placement as shown in the diagram:

 

If the servo is then placed is before FAM7821, and then delivers more fluid pressure through the limiter would this affect normal braking under normal circumstances?

Or have I got this wrong and there is the same fluid pressure output from the servo and there wouldn't be any difference then and the limiter would behave the same regardless of servo position/no servo?

I have heard that servos just help with pedal pushing (less foot pressure needed), and does not 'amplify' output pressure - is that correct?

 

Thanks again.

Edited by lildeucecoop72, 13 May 2022 - 07:57 AM.

[Ethel]

The FAM valve works by the front brake circuit pressure pushing against a spring. When the pressure  compresses the spring enough, it closes a valve blocking off the rear brakes so increased pressure from the master cylinder can't reach them. The maximum rear line pressure is a factor of that spring's strength regardless off how it's compressed. If the Servo was after the FAM, then there could be proportionally more (servo assisted) effort going to the front brakes than is acting on the spring. I suspect it wouldn't make a huge difference,  but it would increase the likelihood of a rear end skid if it had a sufficient effect.

 

A servo uses atmospheric pressure acting on a diaphragm to push a piston. That can be the same piston in the master that the pedal acts on. There it's controlled by an air valve linked to the pedal's push rod. As you press the pedal the valve allows atmospheric pressure in and the diaphragm moves until it catches up with the pushrod & closes the valve. When it does so, the piston has the force from the pedal plus the force acting on the diaphragm acting on it - it's proportional because the farther you push the pedal the more atmospheric pressure is let in to close the air valve....

 

A remote servo is essentially the same. Obviously in needs its own piston, with a push rod connected to its diaphragm, but there's also fluid behind the piston that transmits the force coming from the master cylinder & the air valve is operated by that instead. As I think I said previously, when the servo adds additional oomph its piston moves, increasing the volume of the cylinder behind it: that allows the the plunger that operates the air valves to return to its rest position &the air valve closes.

 

 

I could have just said the harder you push, the harder the servo pushes too 

 

 

 

It's the size of the diaphragm what matters, for the ones we have they about double your effort, the remote one does 1.9 IIRC.

[lildeucecoop72]

If the Servo was after the FAM, then there could be proportionally more (servo assisted) effort going to the front brakes than is acting on the spring. I suspect it wouldn't make a huge difference,  but it would increase the likelihood of a rear end skid if it had a sufficient effect.

 

OK did you mean before the FAM here? As the servo after would not affect FAM, or did you mean the spring in the servo?

 

 

 

I could have just said the harder you push, the harder the servo pushes too 

 

OK so if its before FAM then this will/could affect normal braking?

 

 

 

It's the size of the diaphragm what matters, for the ones we have they about double your effort, the remote one does 1.9 IIRC.

 

Correct, I have the AP Lockheed Remote servo:

 

http://www.minispare...|Back to search

 

 

Thanks Ethel, there were a couple of points here that weren't clear to me. See above.

[Ethel]

Said spring is inside the aforementioned FAM doo-dah. 

 

Perhaps the best way to think about the servo is it will multiply whatever force you put in to it, just as if you had thighs like Arnie. If you want things to operate just as they did before Jacob Rees Mogg got out 'n Mr Schwarzenegger got in, put the Servo first after the master cylinder.

 

 

The yellow band master also increases the front braking effort relative to the rear, but no modification was deemed necessary (about brake proportioning) when it replaced earlier masters that put out equal pressure F&R.

[Steam]

Just as a side thought, having recently removed the remote servo I did not notice any difference, the brakes feel and operate just the same with or without the servo.

[lildeucecoop72]

I ended up not having the servo at all and changing my rear slave cylinders that were meant for this set up (smaller bore than the ones I had before).  I found Keith Calver's site a helpful read (rear bias adjustment) in this regard.

 

As Steam mentions, after trying the setup with no servo I can't feel any difference from the old system i had with the servo in place, so either the new MS and setup  as intended were designed for not having a servo, or my servo never worked in the first place.